Hi Mike,
You'll have a wonderful time here. Spring in London is fantastic.
Yours is of course a very open-ended question are there are literally thousands of restaurants in the capital to choose from.
I'm a huge fan of
Strada. It's a chain of Italian restaurants with pasta, meat dishes and pizzas but doesn't have that chain feel; every restaurant is decorated very nicely and in the most modern of styles. There are several in Central London, including one right off of Piccadilly Circus and one just across the river from Embankment station.
I'm less of a fan of the
Pizza Express chain (very similar to Strada and more ubiquitous, although the pizzas are much smaller than Strada's but the same price) but you could do much worse. They are everywhere.
Nando's is a chain of chicken restaurants but they are actually really good and high quality. They also have veggie dishes. All of their food is grilled and actually quite healthy. Nando'ses are everywhere, too.
As suggested above,
Wagamama are quite good and all over the place. I'm of two minds about Wagamama. They can get really busy and hectic and if you're unlucky, your dish won't be filling at all. But generally the flavours are good and it's very decent value for money.
These are all definitely
not tourist establishments as I go to all on a fairly regular basis and wouldn't be caught dead in a tourist restaurant.
I would avoid any "steak house", especially one with "Angus", "Aberdeen" or "Scotch" at the beginning of the name. Pure tourist traps.
I'll keep thinking and let you know what else springs to mind.
It'd be great if you knew the areas where you'll be spending the most time - in the West End? In South Kensington? Somewhere else?
Cheers