Cheap Meal in London

machofairy

Not many London restaurants are cheaper than the infamous Wong Kei near Soho.

Good Chinese food plus free unintentional entertainment watching the rudest waiters in the UK at work!

hoyin

Don't take your girlfriend to Wagamamas on her 21st!!!!
It's a canteen-style place where you share tables with lots of other people - nice food, but hardly glam or romantic.  You could always go for lunch on another day if you are staying for the w/end and use the BOGOF.
I also agree with all those who say stay clear of Kings Cross for food.
But there are two nice romantic French-style places a short bus/taxi/tube ride away (or you can walk in about 30mins):
Little Bay (especially fantastic value before 7) on 171 Farringdon Road (you could get tube to Farringdon, two stops from Kings Cross)
http://www.london-eating.co.uk/2710.htm  
OR
Le Mercure on 140a Upper Street
http://www.london-eating.co.uk/2352.htm
As someone else mentioned there are a lot of good places on Upper Street - and it's a good place to end up for after dinner drinks too.

Yup I second that Little bay is good and romantic .. I think your girlfriend would be impressed, plus if you go before 7pm they offer a good discount. It is not too far from Kings Cross either. I think a black cab would be around £5. Or take a bus.

Le Mercure is quite expensive and always booked, so you will need to book in advance to ensure a place. I have been a couple of times and did not think that highly of it though.