Do you mean one exclusively for UK people, or one about UK ponies?
Due to the whole international nature of the internet and the pony community, I'd be very surprised if someone had set up a forum where only UK people were allowed to post. You seem very UK-centric - foreigners don't bite you know!
The MLP_UK list is pretty informal and mostly chat rather than trading. But it has a nice atmosphere. It's one of the least flamey pony places I've found And if you had ponies you wanted to sell in the UK I'm sure you could post them there.
Though the thing I love most about the TP is that it's global. No better way to meet people you'd otherwise never meet Even if you can't/dont want to trade or sell abroad (and that's a shame, if it's so, because you're missing out on a lot), you can still get to know other collectors in other countries The first ever trades I did on the net seven years ago were with American collectors and they were all star people to deal with!