We are flying in May from Boston to Barcelona via LHR on BA, and I'm trying to figure out which flight will allow enough time to make our connection. We have one checked bag. We will have to go through immigration, but will we also have to go through security again? Are we likely to have to change terminals if they're both BA flights? Available connection times are 1 hr 45 min, 2 hr 45 min, 4 h 10 min. Which do you think would be the best connection time, given current conditions at the airport?