I will be travelling on a UK passport from France. I know I need to clear I migration, I am not sure about customs and whether or not I need to recheck hold luggage. I know I need to get from terminal 5 to terminal 1 and go through security again. I have no idea whether I need to go through customs, with or without a checked bag, and I am aware that I have very little time. Is this something you can help to expedite?