Do you mean ready made sushi to just buy and eat it? They sell that in the sandwich section in Boots and Sainsbury's for sure cos I have ot for lunch sometimes, they do a itty bitty one for �1 and it's lovely.
You can get it most places these days, being a bit of a sushi hound I have tried them all. Tesco's, Sainsburys, Marks and Spencer, ASDA all do it as mentioned before normally near the sandwiches, also Pret a Mange do a good selection if you have one near you. But mostly its a poor imitation of the real thing because the key to sushi is freshness. Not sure if they still do it but Sainsburys used to sell Yo Sushi boxes which are by far the best.