This is what Timatic says (for NL->IN with transfer in UK, Indian citizen, NL residence permit holder):
TWOV (Transit Without Visa)
Nationals of India with a normal passport transiting through London
(LHR) or Manchester (MAN) with a confirmed onward ticket for a flight
to a third country on the same calendar day. They must:
- have a common format residence permit issued by Netherlands, and
- stay in the international transit area of the airport, and
- have documents required for the next destination.
Nationals of India with a normal passport making a landside transit
with a confirmed onward ticket for a flight to a third country that
departs before 23:59 the next day. They must:
- have a common format residence permit issued by Netherlands, and
- clear immigration, and
- have documents required for the next destination.
Warning: Transit through the international transit area of Manchester
(MAN) is possible between 5:00 and 21:00 on the same calendar day.
Basically, Timatic agrees that you don't need a UK visa to make an airside or landside transfer in LHR or MAN (limitations apply) if you have a common form residence permit (so, a residence permit card, not a D visa or a temporary residence permit equivalent, like Fiktionsbescheinigung).
As a EU resident that needs a visa to enter UK, this was my understanding of all information I read for myself in the past too. Just to make sure, I'd suggest bringing a screenshot of the UK gov page you're looking at with you in case your airlines does not want to let you board.