It may be a little bit too late for this year but I've just put together a website of UK mail order suppliers of plants and seeds.
There are other lists of lists, I know but this one is a bit different in that it:
only includes UK firms
only covers mail order
includes vegetables - 4 categories: straight veg seeds, exhibition seeds and young plants, heritage seeds and finally potatoes.
There's also organics, green manure, herbs and lots of ornamentals
www.plant-seed-mailorder.org.ukThere's a site search in case you're looking for something specific
Any comments - good or bad would be appreciated
Phil