I believe supermarkets and DIY chains (like B&Q) sell seeds which are left-overs from commercial tomato seed sales. They are bred to be machine-picked (i.e. thick skins) and all to come into fruit at the same time (the opposite of what garden growers want). So buying supermarket seeds is not much better than buying supermarket tomatoes - the are the same. I think apollo's suggestion is a good one - look for small specialist suppliers stocking heritage seeds. I try to remember to de-seed my breakfast tomatoes in Turkey, leave them on a paper napkin in the sunshine to dry out and then smuggle them back to the UK. Do Turkish tomatoes grow in the UK? They do on a sunny year - and in big pots (say 14 inch diameter).