I've just found that
http://www.mr-fothergills.co.uk
(http://www.mr-fothergills.co.uk)
has a selection of vegetable (and flower) seeds at half price. Best value, I think, are the St Valery carrot seeds at 2000 seeds for 68 pence. The postage for seed orders is free.
If you take the link from the homepage, there's no need to trawl through all the vegetable seed pages...unless you want to. ;D