Chipper, my dc are MASSIVE my little pony fans! I think the mane (get it? hehe) thing to find out is which ponies if any they already have. If they don't have any yet I'd go for the main six who are Rainbow Dash, Apple Jack, Pinkie Pie, Fluttershy and Twilight. I think they're on offer in asda at the moment (£3.50 iirc). Another option is Equestria Girls who are the human versions of the ponies. Again I'd go for the main six characters first. There are three equestria girls films, if you go for the dolls from the older films you'll probably get them a bit cheaper. The newest film is Friendship Games and the dolls from that film seem to be a little dearer.
Another option might be a mlp busy book. Asda and tesco sell them. They're only £5 and you get 12 pony figures, a playmat with various scenes and the book itself. Really good value imo. Both my dds have it and love it.
The mlp magazines are £3.99 (I think, somewhere around that mark anyway) amd they sometimes have really really decent gifts with the mag. This month is a limited edition nightmare moon figure. I bought the dds one to go in their stocking but then realised it was a special halloween mag so ill keep that for halloween instead but it's definitely worth keeping an eye on what toys come free with the magazine each month.
Another option could be one of the equestria girls films on dvd. I saw them in primark last year for about £5, I'm sure I saw them on amazon for the same price. Although the newest film would be more expensive, I think it's around £10. My dd's favourite is the second film, rainbow rocks.
Anyway, I have rambled on just a little too much about mlp and have just realised that I really need to get a life! 