Have a visa question and would love assistance here:

Traveling LHR-BOM-CMB on a combo of British Airways and Sri Lankan (different PNRs but both Oneworld). 4 hour layover in between.

However, the return I think might be an issue as it's CMB-BOM-DEL-LHR. CMB-BOM is again Sri Lankan but the second PNR which was ticketed by BA is BOM-DEL-LHR with both Air India and BA. Do I now need a transit visa because of this routing? I wish I didn't have that extra AI segment but alas, it was the best option on BA's website and they seem to interline.