The question I was trying to answer was about a Filipina travelling from SE Asia (possibly from Thailand rather than the Philippines) travelling to Norway with a change at Heathrow involving 20 hours on the ground. Getting any sort of visa for the UK is extra stress.
I couldn't work out how to drive the IATA page to find out about transit visa requirements. It only seems to know about landside transit, for which a Filipina would definitely need a visa.
The Home Office web pages were no help. In the case of the link offered, it rather assumes that one intends to enter the UK, whereas the plan here is not to enter the UK, at least, not landside.
Thanks for the attempts to help, but I was hoping to find someone who knew the answer.