When it comes to spices in British cuisine, you can read some earlier answers here on the topic. In this thread u/Noble_Devil_Boruta describes what Europeans used spices for and why there aren't many in "traditional" European dishes and here our u/bitparity looks at it from a longer perspective and also mentions non-culinary use of spices. In this response u/GrunkleCoffee has a rather modern answer for the question of why British food is bland. In addition you can read this thread about the history of the English East India Company and the beginning of British influence on India, which may partly answer your second question. Further responses by our users are highly appreciated, as always