Britain has a long history of assimilating food knowledge from other countries - we didn't really have any knowledge of cooking until the Romans invaded. Since then we've welcomed a lot of different cultures in and adapted them to suit what's available in our climate. What you would find in an Indian restaurant in London is very different to what you'd find in Delhi.
To claim that Indian or Chinese restaurants in London are not part of British cuisine sounds a little odd to me. Any food you see in London, whether it's boiled mutton or sushi, is inspired by another country somewhere along the line.