Yeah, there is nothing saying that the starting point needs to match the country where your BAEC account is registered.

Somehow I think it may be better to avoid my LHR-CAI-LHR run. An ex-AMS trip would seem to be a good alternative (except for the inevitable LHR-AMS hop...). MIL is not bad either: NYC or BOM in CW are 1,573 EUR return (less than £1,350)!!