My parents went to the first Chinese restaurant in Oxford when they were courting in the late 50's, it was exotic and very popular.
The first Mexican restaurants were a novelty in the 80's
I first went to Thailand in 1988 and there were no Thai restaurants local then. Now they are common as Mxican. We even have a take away run by ex-Gurkhas now. I'm sure all of these things started in London well before, things tend to slowly trickle out to the provinses.
Indonesian food is common in Holland but not here because Britain never had Indonesian colonies like the Dutch did.
As the other guy said, Indian food, particularly chicken tikka massala is probably our national dish now.
More exotic foods came to be not just because of immigration, but because there was the boom in foreign travel for ordinary people which started in the 70's and this created the demand for it. This also allowed for the import of foreign ingredients. I first had sweet/ bell peppers in around 1975, very exotic then.
We can get pretty much anything and everything in a regular supermarket now.