I live in a one-horse town (all of my "local" toy shops are a good 15 mins away *minimum*) so keep that in mind... Also I don't know any kids nor do I really pay attention to them while I'm out, but I don't recall ever really seeing any kids in the pony aisles when I've been there. Again, that might just be me not noticing them, but I do remember seeing kids looking at MHs, Barbies, Sylvanians... Ponies? Nope. I assume kids DID like them but I don't have any personal anecdotes there or whatever.
G4 - distribution seemed good at the start of the line, but I wasn't really paying that much attention to them at that time x) didn't start collecting G4 really until around the wedding singles (Trixie, Lyra etc), so up to that point I'd just notice them in passing while looking at other toys, or just glance at them to see if any caught my eye, etc etc.
mid-G4 up to the movie - generally well distributed, blind bags were a pain in the butt to find once they started splitting the waves into two (one half of the wave tended to hang around for ages, finding both halves was annoyingly difficult), never saw the last couple of blind bag waves at all (go fig - I really wanted those!!).
store-exclusive sets tended to NOT appear over here. i.e. the big set with Haven Bay, the TRU exclusive sets with the cool chars like Chrysalis, Lyrica, etc. (I think I heard the TRU multipacks DID show up at some specialist shop, but I never saw them in person. If they did then that shop was nowhere near me, lol.)
FiM Collection Apple family sets shelfwarmed for years and ended up at pretty much every discount shop in the country. I recall the Power Ponies shelfwarmed quite a bit and ended up bouncing between shops, too, but not quite as badly as the Apples. They definitely ordered waaaaaaayyy too many of those. (I suspect it might have been a case of them just being made too late? They were all based on eps that were years old by that point... makes me wonder if they'd have sold better if they'd been released when they were actually "relevant". Didn't matter to me since I don't watch FiM and was just happy to see more characters, but I do wonder if that played a part in them not selling well...)
The other FiM Collection sets.. IIRC the one after that (Pinkie, Maud, Gummy etc) got normal distribution and did not shelfwarm. Nightmare Night never showed up over here. (the bat pony knight might have shown up in TK Maxx because I do seem to recall buying him in a shop... but I had to get Ditzy and Luna online, so definitely most of them didn't show up here) Rarity set showed up at Toys R Us only I think (and not too long before TRU shut down, either).. believe the Canterlot ones only showed up in discount shops once or twice.. never saw the 2018 ones.
Equestria Girls - great distro for the original line, don't remember any problems there at all really. some of the later dolls only showed up at one or two shops very briefly, but _in general_ the line was well-distributed, as I recall. The reboot dolls I think only showed up at Smyths Toys and I recall them shelfwarming for quite a while. Minis were middling, some showed up, some didn't.
Agree w/ Guardians of Harmony shelfwarming over here and some of them just not showing up at all.
G4 Movie line and the accompanying Reboot ponies - that's where distribution started to get funky. The basic main 6 brushables were easy to find, the new chars (Soarin, Lyra etc) I don't remember hanging around for too long so they were probably very popular. Sia pony shelfwarmed like crazy, think my Entertainer only just managed to get rid of their stock of her this year. GoH Movie sets (with the pirate birds, etc) are STILL hanging around some Entertainers, those really didn't sell over here. Never saw the Capper set. (Can you guess who was the only movie non-pony char I wanted a toy of?
) Nor the hippogriffs - had to get those online too.
Baby hippogriffs, I only saw them ONCE, months after I'd broken down and bought them off eBay, at a small "mom n pop"-type local toy shop. No idea where that guy got them. The baby sea ponies I remember only showing up at discount shops, but they were very plentiful in them and stuck around for a while. (Wouldn't say they were shelfwarming)
I'd say distribution for the Movie and Reboot ponies was about 50/50. Obviously some sets do just slip through the cracks, that happens with pretty much every toy line, it had always happened with G4 (well, with MLP full stop!), but at this point, more stuff wasn't appearing in the UK. (Or if it was, it wasn't making it over to my neck of the woods!)
Most of the last few G4 sets just didn't turn up at all.. pony sections started getting smaller and smaller... mostly you'd find newer stuff at TK Maxx or The Entertainer, if you were lucky, once in a blue moon, and usually they'd be missing half the set. (this is why I'm missing some of the Retro Rainbow set.. *pokes sig*) I.e. Sandbar never turned up over here, but I did find him at TK Maxx the other day. *Edit: Just remembered these - the TAF-esque ponies (I forget the actual set name, Cutie Mark something maybe?) never showed up over here at all. Found that set in Zurich though, when I went there back in Feb 2019. But yeah that set skipped the UK entirely.
Also want to add that even the G4 reboot stuff, esp. later releases, that WAS released in the UK, a lot of it did not stick around on the shelves for very long. Especially if it was a non-main 6 set, it was very much a case of "grab it when you see it, or risk missing it".*
G4.5 - God help you if you wanted these. ASDA and Smyths got one wave of the potion ponies (I forget which one) and the Fluttershy/Pinkie dress-up ponies, they shelfwarmed for months. ASDA got wave 2 of the potion blind bags and they've turned up at some GAME stores recently. Entertainer has one wave of the crystally blind bags (I dunno which one) and I think ASDA might have them, or might've had them in the past, not sure? And all three had the Smashin' Fashions singles. they're still shelfwarming at Entertainer (I was looking at them just today) and I believe Smyths still has some of them too. then the only other G4.5 I've seen in shops is the big hairplay Cadance (with horribly pixelly-printed eyes!).
Oh and those EQG mini fashion pack things showed up around this point, I dunno what gen those are part of. The ones with Minty and.. I think Cadance? They hung around for ages. hell I think my Smyths might still have some.. know they still have those tiny lesser-detailed minis on the cards..
The pony shelves at this point were TINY everywhere, a lot of places just didn't carry ponies any more. Don't think Tesco ever got any of this gen in.
G5 - Hard to say since we're still in the early days, plus COVID's messed things up to no end. G5 didn't show up here until months and MONTHS after they first came out in America. Not much variety yet. Pony sections are still itsy-bitsy. Entertainer hasn't got any yet, ASDA and Smyths have them... I was in Hamleys last week (in London) and they had no ponies at all. Now I can honestly say I don't think I ever went to Hamleys in G4's hey-day (if I did, it was too long ago for me to adequately remember!) but I imagine they probably had some ponies back then! Don't think Tesco has these yet but I'm blanking...
Not many different ponies available over here yet. I'm looking at MLPMerch's G5 page right now... we have the scene packs, the big Rainbow Reveal Sunny, the singing Pipp with the fluffy wings, think we have the zipline playset too?? And that's it, I think. That's pretty much the entirety of the pony section now. That and shelfwarming G4.5/EQG minis.
The crossover ponies like Plasmane, D&D ponies, etc (what gen are these considered?) - basically nonexistent over here - only ones I've ever seen in person are the D&D set at Forbidden Planet in London (and they were marked down on clearance when I saw them this summer - just 4 or 5 sets scattered about on a low shelf). Can't speak to whether or not they flew off the shelves since I only saw them that one time (was there on a day trip).
Basic Fun distribution - terrible and horribly inconsistent over here. Let's see.
Collector ponies - Forbidden Planet had these briefly, they also had the Pretty Parlour set for a hot minute. Smyths also had the Pretty Parlour for a decent amount of time.
Unicorns/Pegasi wave 1 (Glory, Firefly etc) - think these were about the best distributed of the lot, I recall Entertainer and Smyths had these, like 90% sure John Lewis got them too?
Rainbow wave 1 - Don't think I ever saw these personally, I might be misremembering though. None of them look familiar to me. I definitely don't think we ever got the scented ponies.
Rainbow wave 2 - John Lewis had these and they shelfwarmed for a while. After that, they gave up on stocking ponies entirely. Don't think any of the other shops got them in but could be misremembering.
Earth ponies (Applejack, Bow-Tie etc) - Never saw these. I understand they DID show up at some Toymasters, but I couldn't get to one to see for myself.
Stranger Things Applejack - I caught a glimpse of this one at a Toymaster that I was in for about 0.5 seconds before it shut. x)
All other Basic Fun releases - Never seen them. Haven't seen any in shops for a long time. Most of the ones I did see seemed to shelfwarm.
I will note that the Funko POP G1s, they haven't shown up in a lot of places (Never seen them at any Forbidden Planet; I've only seen them at Entertainer and ASDA, and ASDA only got two in.. I think CC and Butters) BUT they did seem to sell out quickly at ASDA, at least. My Entertainer doesn't seem to be selling them, but they tend to get stuck with POPs, especially "retro" characters, for a LONG time (they still have at least one Helga from Hey Arnold and she was released in 2017!), so they're not exactly a great point of reference x) But yeah, they didn't hang around in ASDA for very long - which granted I don't think ASDA got very many of them in the first place, but they certainly got *some* in and I only bought two, so... someone bought 'em, lol.
I rambled a lot but yeah tl;dr the pony sections are tiny over here and there's like no variety. I dunno how well they're selling.
(Maybe I should have said I live in a one-"pony" town...)