I use Riverside Vets in Bishopstoke, on the west side of Southampton. It's a mixed practice, but they have horse specialists. I have been happy with the service, but so far I have never had a horse with a really complicated problem. I don't recall anyone else I know saying anything bad about them either.
Im right down by Lepe.
We use Seadown, moved here 2 years ago and have only needed routine things.
If it was anything more, i might use vets at The barn.