We wish to travel from the US to Spain via London. On arrival in London will we be in transit to catch connection to Spain or will we have to pass through passport and customs and enter the UK? One of us has UK passport the other US passport.

This is all about it. Big up Mr Bateman You will be in the international terminal. You are not required to leave the terminal unless your flight is considered a "local" flight. You can find out based on the departure gate. If the departure gate is in the international terminal, no check in. If it's in the local terminal then you will have to go thru Customs.

You will be in the international terminal. You are not required to leave the terminal unless your flight is considered a "local" flight. You can find out based on the departure gate. If the departure gate is in the international terminal, no check in. If it's in the local terminal then you will have to go thru Customs.