WAV, there is nothing new with the phenomenon of 'going out for an Indian' (to put it one way). Eastern and Asian influences on English cuisine have been enormous. Tea, a tradition picked up from the East. Kidgeree, mulligatawny, Worcestershire sauce, all adaptations of Indian cuisine. Then come things like coronation chicken, dishes influenced by the East. Curry, as a spice, owes as much to Britain as to the Indian subcontinent.
Hannah Glasse, way back in the 1740s, included elements of Indian cooking, or what she thought was Indian, in her cookbook.
Curry appeared on menus in Haystreet in the 1780s and in the early 1800s, Dean Mahomet opened the first Indian restaurant in Britain, to serve faux Indian food which would appeal to those who had served in India. The main reason it failed was that they brought back their own chefs to cook Indian food!!!!