You can do this in either direction, on separate or same PNRs. No problem at all. I’ve done this multiple times on Chinese passport which has the same UK rules for transit as for Indian passports.

the only exception is Qatar - QR will refuse to board you if you cannot enter the last destination on QR ticketed point as a final destination. As long as you don’t fly QR you are good.

And I can assure you that whilst airlines might consider other separate tickets in their assessment of whether you have the right to enter your immediate ticketed destination or your separately ticketed final destination, contracturallly they only have to consider the journey you have presented on the ticket that they are operating. Anything else is a customer service gesture or possibly a customer policy.

I can also say having travelled with a Chinese national that Qatar have considered onward travel ticketed with BA when their own ticket ended at LHR. But that is a published Qatar policy.

YMMV.

The UK rules are extremely clear. OP will have no problem. In fact anyone with a US or Canadian visa/permanent residency card can enter the UK (clear immigration) without a UK visa for up to 24 hours and are even permitted to travel out of a different airport in the same city (LHR arrival and LCY departure for example), if they are arriving into the UK directly from US or Canada or having a ticket from the UK to US or Canada. Many people, including often airline employees, don’t believe this but it is true and available. It’s published on home office website and also in TIMATIC for airlines to view.

Update on my experience.

I had no problem checking in at JFK. The agent asked if I had a UK visa & I told him I was transiting on the basis of my US visa. No further questions were asked.

I landed in LHR & went for immigration. The officer asked me when my next flight was & I told him I had a booking for later that day as well as another one for the following day depending on his discretion to let me in. He said I was permitted to stay till midnight on the following day. So I went into the city, met up with some friends, stayed the night & left for CDG the next evening.

Thanks for the update and I'm glad you had a decent officer who let you in!

I had a quick 30 second discussion with the officer regarding this & he said that the US visa makes the transit eligible for almost everyone & the only reason they would deny entry is if the passenger was flagged in their system.

I got back to DEL yesterday & this morning I get an email letting me know my UK visa has been approved & I need to submit my passport for stamping. :p

They took 15 weeks to issue the visa & did so on the day after I got back from the trip where I was supposed to use it. Talk about timing.

Apologies for the incompetence and under resourcing of the onmishambles that is the UK Home Office, it really is a disgrace to the UK.

It is what it is. I have trips planned for October & December so I will be able to use the visa in the end but they definitely have to get their house in order.