Today I had lunch at the Indian YMCA which is on Fitzroy Square, near Tottenham Court Road. The nearest tubes are Warren Street or Great Portland Street.
This is the canteen of the Indian YMCA and mainly caters for residents but anyone is allowed to walk in off the street - the canteen is just to the right of the street door. The decor is inoffensive, it looks like any other office or college canteen. The food is all self-service from a slightly cramped serving counter. There is no alcohol and no smoking, and you get free tap water to drink.
The menu is limited, you get curry, curry or curry - no choice of vindaloo or korma! The curries are pleasantly spicy, but not particularly strong. They also offer rice, chappatis and a few sides like chutney and papadums. There are several veggie options too.
We had: two rices, one chappati, two papadums, one dip thingie, two meat curries, a dhal and a chickpea curry. With hindsight this was too much and we could have skipped the dhal (which was rather bland). The total cost was £10.40 :)
I think that qualifies as the cheapest meal I've eaten in central London!