I believe it is standard procedure for airlines at LHR to hand over lost property to HAL lost and found, so I think that’s your best bet - looks like they even have an online inventory of all items:
https://www.heathrow.com/at-the-airp.../lost-property
That is of course assuming the cleaners were thorough enough to find it during the turnaround - if not it may have made it back to BKK and beyond.
Fingers crossed for a positive outcome !