Must recommend Wawa on tower bridge road. Slightly odd location, but cannot fault the quality of the food. Have been going for a few years and it has been consistently good. In terms of price, it has got more expensive in recent times, but feel it is justified as I haven't found anything else that matches it in SE1
I haven't been to anywhere in SE1/SE17 that does good sushi.
There are the chains, and there's WAWA on TBR (overpriced and not really a sushi place in any case - despite doing a few rolls). There's Bon Bori, but again that has limited sushi options. And also Feng Sushi - again, not really a sushi place.
There's Ichi at the Park Plaza. I've never been there.
For value and quality, I'd go into town (or up to Swiss Cottage) and go to Atari Ya. They import nearly all the fish for nearly all the top sushi places in town in any case.
There are other places for good sushi, but you wouldn't necessarily describe them as cheap.
I also recommend Atari Ya, they are run by Japanese who know their sushi. I used to work for a Japanese trading company and we supplied them with the best fish and seafood you could buy. They have proper deep freezers (that go down to -50degrees C) as it is done in Japan so that the tuna does not to degrade. At the time, they were the only ones to have these freezers in the UK.
This may not be what you're looking for, but there is a sushi kitchen at the apex of Kennington Lane & Kennington Park Road that offers home delivery (24 hours notice) and sushi chefs for hire. Worth knowing about if you are entertaining at home/work.