BOM same day turn
 

on BA LHR-BOM-LHR (BA 139/138), is a 2 hour turnaround time enough? is it the same plane going back?

It is the same plane that turns round and heads back to LHR. It's tight but assuming no checked bags and you not leaving the airport, you ought to be able to make it through immigration & customs and up to departures to check-in for the return in that time.

The flight time is about 9 hours so you ought to be able to OLCI for the return flight on BA138 whilst waiting for the BA139 at LHR. This should enable you to go through flight connections at BOM (cutting out customs etc) - but only if you've already checked in, as I understand it. And again assuming you don't have checked bags and don't need to go landside for any reason.

The LHR-BOM-LHR flights (timings and distance flown) are the most perfect combinations for aircraft scheduling. Only one aircraft. about 2 hrs turnaround at both LHR and BOM.

If you cut out immigration and head to flight transfers (BOM intl terminal is pretty medium-small sized compared to others), 2 hours should be plenty and perfect. I suggest you get an Indian Visa as BA may not let you board in LHR even though you doing a LHR-LHR via BOM (India also operates APIS!).

good luck and let us know if you try it!