I did this (although routed BOM not MAA) nearly two years ago and you don't need one if as you say you're staying airside and the connection is under 24hrs. I found that if you have your onward Boarding Pass printed at LHR it was hard to get into the Lounge at BOM. I was turned away by the staff at the UL Lounge despite flying in J with them and having a silver card apparently because my ticket was printed by BA, had the Logo and British Airways written on it. I was eventually rescued by the Special Services lady who got me into the shared lounge BA use, having spoken to UL who said they were happy to pay for me to do so. I hope you have better luck at MAA.