Thanks everybody. Yes flights LHR-BOS-LAX are already booked. Only one big suitcase actually. Just wasn't sure as, if I am not mistaken, on BA's EU hand luggage only fares you can't check luggage for free, even with Silver status. So didn't want to get caught out over there. But it sounds like I'll be alright with up to 2 pieces at least. Also I'll be staying overnight in Boston so will have to check-in there again anyway.
And yes, it's Compass Air for American Eagle... Anyone happens to know if those earn BA tier points?