In the UK, any food product, including soya, must be clearly labelled if it is derived from a genetically modified organism, but many people worry that products not labelled as having GM might still be contaminated. I don't want to eat GM food, so I believe a good way to make sure is to buy organic tofu, simply because the organic sector best polices the movement of GM-contaminated material. Trading standards offices in the UK are particularly concerned about fraudsters passing off non-organic foods as organic.
I believe you can get 250g slabs of Cauldron tofu in Tesco, Sainsbury's, Waitrose, Morrisons, Co-op, ASDA or online from Ocado (owned by Waitrose)
[EDIT] crossposted with cheesebiscuit - if you don't like Cauldron then you can get 280g packs of Clear Spot organic tofu for £1.53 (at the moment) from Goodness Direct and if you prefer to go to a shop Google for your nearest Holland and Barrett store, or for health food stores in your location.