I buy some from each of several suppliers. T&M tend to be expensive but there are some bargains. Mr Fothergill and DTBrown both keep pretty much the same stock but seem to take it in turns to have cheaper and more expensive ones eg a certain variety of tomato will be cheap in one but expensive in the other, whereas a chilli will be the other way round (I think they are owned by the same company now). The Organic Gardening Catalogue has mostly organic seed, but lots of it is from the EU which doesn't necessarily do well in the UK. Marshall's tends to coat their seeds in antifungals which I don't like. I'll just go and look up the name of a little company in Wales.......
The Real Seed Catalogue
www.realseeds.co.uk . That is a gem - a fairly small range but many seeds produced in Wales, varieties which do well here and the price is good as it's jointly owned by the customers - or something (they explain)
When looking through the catalogues, check the seed counts - they are very low this year.