Agree that 5 hours between landing and push back does not give you enough time to clear immigration, meaningfully visit anywhere, re-clear security.
Yes, you could certainly get the train to Paddington, but why? The area around it is a hole. Yes you could get the underground somewhere else, but only really with enough time to pop your head above ground before heading back.
And let’s not start on the possibility of disruption to the trains!
PS - just be glad it’s 5 hours at LHR, and not 5 hours at LRH (as per thread title); having recently spent 1 hour at LRH airport, that is more than enough!