The answers are there if you actually bothered to look.
Whoops! The problem is that if the scheduled wait were 30 hours, I'm very sure that wouldn't be allowed.
You told us it was 20 hours - you need facts to get the correct answer !
Either they stay airside or they pass through Border Controls to landside.
Using the answers Philippines, In Transit and passing through Border Controls gives the result:-
You’ll need a visa to pass through the UK in transit
You should apply for a Visitor in Transit visa if you arrive on a flight and will pass through immigration control before you leave the UK.
but getting a 'visitor in transit' visa would be a major hassle.
Why? Hundreds of others do it without problems every day.
Staying airside they don't need a visa, passing to landside they need a 'Visitor in Transit Visa' - take your pick.