As long as I mentioned Tayyab's, I should have mentioned Maedah Grill:http://www.maedahgrill.net/
Not your average kebab shop. Turkish food in a stylish, comfortable setting. Same neighborhood as Tayyab's...so you could conceivably do lunch at one and dinner at the other. Whitechapel or Aldgate East Tube. And the Whitechapel Gallery is worth a quick stroll through (which is about all there is to it anyway).http://www.tasrestaurant.com/
Tas is Anatolian, so think a little bit like Greek and a little bit like Turkish. There are a few locations, one near London Bridge Tube (so, also near Borough Market and Southwark Cathedral) which also has a cafe where you can pick up takeaway items, and another on The Cut near Waterloo Station. This and Tayyab's are probably the more expensive of the choices I've posted, but still within reach...good places to take a date, or a group of friends.
And if you're hungry for inexpensive, fresh made burritos (think Chipotle), you have a few choices:http://www.tortilla.co.uk/
Tortilla has a few locations, but my favorite was in Islington, across the street from Angel Tube station. Refillable soft drinks...what a concept! And frozen margaritas! Shame you can't sneak one out in a takeaway cup...http://www.chilango.co.uk/
Chilango is also good, one location just up the street from Tortilla in Islington, but another in Fleet Street not too far from St Paul's or the Thames.
Mexicali, 26 Berwick Street (and another location near South Kensington/Chelsea as I recall), is by far the poor cousin of the other two. Quality not quite as good, and service which places it firmly in the 'fast food' category; but it is conveniently located at the northeastern edge of Soho...so it's good for a quick bite before hitting the pubs and clubs (closes at 9, I recommend not showing up there much later than 8:30 or the surly staff will give you a hairy eyeball), or during an afternoon of shopping on Regent/Oxford Streets.
A couple more in Whitechapel/Shoreditch:http://www.bigchill.net/http://www.barmusichall.com/
I only stopped in each of these place once, probably around brunch/lunch but as I recall the food was excellent in both cases. Seems to me I had a breakfasty sort of thing at Big Chill, and a Burger (my mouth's watering thinking of it now) at Bar Music Hall. Oh yeah, I guess they also do music/dancing after dark...but I was never there at that time.
Gimme time...I'm sure I can remember a few more...I have lots of fond memories of London, and about a half dozen or so good friends who live there. Where will you be staying, how long, and do you have specific plans?