Originally Posted by jerry a. laska

With a similar timeframe, I have taken the bus to Windsor a couple of times to walk around and have a pint.
Long Layover LHR
If you want something closer to LHR on the tube, then a visit to Osterley Park might be worthwhile.
https://www.nationaltrust.org.uk/ost...park-and-house
6 hour layover at LHR
Some other ideas here:
Help me plan my 10 hour layover at LHR
5 hrs at LHR between flights: What to do?