I have an upcoming roundtrip on Saturday OSL-LHR//LGW-TRN with BA.

What is the cheapest option to connect between LHR-LGW-LHR?

I have at least 3,5 hours to make the connection (5 hours on the outbound and 3,5 hours on the inbound).

Tried to do a search, but did not find any relevant information.