I like Burwins on Essex Rd in Islington. Good guys, reasonable standard, very fair, but its never going to be cheap in london!
I dont use them to do work, but do sometimes buy oil, levers, tubes etc.
They are a bit pricey for tyres IMO. But lovely guys.
Bob Thomas in Finchley is ok too. Another good old fashioned shop.
Depends where you are in the city. Get solid quotes before you get work done!
cheers
a