You don't want one itinerary there, you want two.

Booking the LHR-PER and BOM-LHR as open jaws will significantly reduce the cost. You can then add a flight, probably not OneWorld, as a one-way between PER and BOM.

For the long-haul itinerary: LHR-KUL-PER and BOM-LHR is the cheapest, at £616.

Premium economy is £1,263 LUX-LHR-PER and BOM-LHR. You could knock of £57 by travelling on to Milan but they you'd have to get back from Milan. Alternatively you can start in London and pay £1,500 on Cathay.

MUC-LHR-KUL-PER returning BOM-VIE is £2,195 in business class on BA. Cathay is £500 more starting and finishing in LHR and arguably better value.

Mixed class, business to Australia, and premium economy on the day flight from Inida is probably the sweet spot, but I can't get it to price.

You then need to add on PER-KUL - it's $186 US in economy (or $282 US with bags, seats and meals) or $980 US in premium flatbed on Air Asia X (but >$300 less if you bring the trip forward or put it back by a day). It's a fairly long layover in KL but still a day flight.

Not OneWorld but you could look at an Etihad open jaw from London as well...