A couple of places come to mind that won't break the bank - and are decent. Try the Woolsey on Piccadilly - which is more of a brasserie then anything else. It's open for b/l/d - and while not cheap; it's comfortable to eat by yourself and read a paper.
I also think Avenue on St James is good - better for lunch then dinner. The food is described as Modern European/British but it's not over the top in terms of price like some nearby places.
I usually find that the mom and pop Pakistani/Indian places are pretty good - you just have to take a chance, sit down and ask what they recommend.