Agree completely about the Tesco muck. Problem is, as I get exposed to more and more of the good stuff my tolerance level for the lower end sushi shifts so I'm getting less and less satisfied with any of the chains.
Take Yo! One of the mixed menu options will cost from between £9 and £12.50
http://www.yosushi.com/love-club/blog/set-priced-menu
Alternatively, you can go to Sushi Hoshino in Holborn and get a sushi set for £12. It wees all over the Yo! stuff. I haven't been to a Yo! Sushi place for years.
If you're OK with the quality of Yo! but want a little bit of an upgrade, there's Yoshino (or Yoshinoya) in Piccadilly. I wouldn't recommend it to stut, slimy sauces galore as they do more of the "international" styles of sushi, but as I mentioned upthread, this kind does work well as "takeaway sushi", it's on a par with some of the more exotic sushi takeouts offered in Japanese department stores. Alternatively, The Japan Centre in Piccadilly also has takeout sushi.
When it opened, Wholefoods in South Kensington had half decent sushi. Don't know if they've maintained the standards.
Since I live in Camden, if I am going to go for sushi (which I don't do very often) I go to Shimogamo. It's a bit salmon heavy, not as refined as other places, but definitely well within my tolerance and enjoyment range. At lunch time they offer a huge sushi platter for £18, I understand the average price for a sushi lunch at a Yo! is about £14. The extra £4 is more than worth it.