John Lewis in the UK is pretty much the only UK retailer I think that is carrying it...and considering there are only about 40 John Lewis stores across the whole of the country not everyone is handy to get to them! The last time I was in my closest John Lewis they were £13 for the set and they had 3 or 4 of them on the shelves.
And of those 40, not all of them stock it yet. Mine still stocks Twilight's Balloon and talking Celestia only, and we have a large branch near us. They also lack them on their online site at my last check.
The problem with their online options is they work a bit like Argos and the silly "assortment" process - so although the picture is currently the Pinkie and Sweetie Belle set...you could get any of the store packs including the Star Dreams one...you just dont know until it arrives! Very frustrating.