Just been wondering about this and wanted to be on the "safe side".

I already have one flight Lisbon - LHR with BA, arriving around 4:30pm. There is another flight I would then like to take, LHR to Barcelona, which hasn't yet been ticketed. This 2nd flight, would, I assume, be on a separate ticket then.

This 2nd BA flight, LHR - BCN, departs at 6:20pm. Would I make this flight on time? If the previous flight was delayed somewhat, causing me to miss the 6:20pm, would BA put me on the next flight or compensate me in some way, or would I have to completely purchase a whole other airfare?