I saw these today in the local Primark but I didn't know anyone was looking for them. Moonstone is weirdly white though, not pale blue. I don't know if that's an issue for anyone...
I probably won't go to Primark again before I head back to London so I'm afraid I can't help - but there are quite a lot of different Rainbow Pony G1 clothing items at Primark at present, so this seems to be their latest theme.