The Veerasawmy, from an old postcard
The Veeraswamy (99-101 Regent Street, London W1) is said to to be the oldest Indian restaurant in Britain. It was founded in 1926 by Edward Palmer, a retired Indian army officer. Palmer is an interesting character. From an Anglo-Indian family, his grandfather founded a Hyderabad banking house in the 18th century, and his grandmother was Indian. His great grandmother was the Princess Begum Fyze Baksh. Palmer's company E. P. Veerasawmy & Co. (note the slightly different spelling) ran the Indian foods section at the British Empire exhibiton in 1924, selling spices, chutneys and curry pastes to the British public- this must have seemed deeply exotic at the time.
In 1936 he published "Indian Cookery: For Use in All Countries". This is a rare book in first edition, and difficult to find, but I see that a paperback edition, published by Mayflower in 1972, is readily available, and there are various other fun editions you can get your teeth into:
Lizzie Collingham, "Curry, A Tale of Cooks & Conquerors", Random House, 2006
Rhona Aitken, The Memsahib's Cookbook, Piatkus Books, 1993
Pat Chapman's "The Taste of The Raj", Hodder & Stoughton, 1997