Going to BCN on BA, you're going to be switching terminals at LHR. Likely flying into T5 and out of T3 ... Switching terminals involves walking a bit to get through the "connections" area, getting down to a coach that drives you over to T5 (~10-15 minutes), then going back through security at T5 before making it to your gate.

In my experience, I'd recommend giving yourself a couple hours between flights (I doubt BA will book a flight w/ less than 1 hr between). For me, that's given enough time to make the terminal switch, get back through security and get a bite to eat before getting over to my gate.