As well as Pret look out for "EAT" who offer a similar style of product but without the smell of herby mayo.
There's a great chain over here called
Wagamama, which is great food at a good price (Main Courses from about £7.50). There's also the
Fine Burger Co. and
Gourmet Burger Kitchen for a better quality of fast food.
You'll find some of the world's finest restaurants offer unbelievable prices for lunches, and people like Gordon Ramsay's "maze" offer a four course lunch for £29.00.