as surprising as it may be, you can get better indian in chicago than london. I wouldn't waste a meal on indian food in london.
I'd go to herods food court, and maybe the savoy grill, or maybe just a good pub for a meat pie and a schoolboy pudding and a couple of pints.
if you don't want english food, have chinese - the hong kong/ cantonese food in london is differnt from chinese food that you will find pretty much anywhere else. If I were eating one meal in london (and that actualy happens to me pretty regularly) I'd go to poon and co in china town and have the crispy beef, a quarter crispy duck and some noodles. all very hong kong dishes, and very representative of london chica town food.