There used to be a monthly record fair in Southampton on the last Saturday of the month - small, but free entry and sometimes you'd find something good. It used to be in the Star Hotel in the High St. I went to it a couple of years ago but haven't managed to get back since, but looking online it looks as though the venue has moved, and the dates have changed. I think this is the same fair I went to: http://www.usrfairs.co.uk/
I see there are also some fairs in Winchester and Andover. Whether it's the same dealers at each of them I don't know, but hopefully you'll find something you like.
A second-hand shop opened in Southampton last summer. I've not been able to get down there, but it would be worth checking out if you're in town. It's called Back Tracks. Here's a listing with details: http://www.vinylhub.com/shop/12394-Back-Tracks
There's also an Oxfam Record shop in Southampton (in the High St, just up from the Star Hotel) and one in Winchester, in the upstairs of their book shop.
If you're in the area, there's also a new 'new' record shop in Romsey, which is just a few miles away. It's called
Hundred Records. Again, don't have experience but it would be worth checking out if you're there.
Let us know what you think if you visit any of them.