At Heathrow
Flights before about 08:00, and arrivals so late the centre would be a closed Tube and a long taxi.
You can still ride in for the day and come back. Confirm which terminal the hotel actually connects to.
LHR · Where to sleep · published · checked
Where to sleep on a London layover
Boring is the aim. A famous postcode that strands you is the mistake.
Paddington
The Express hotel. Fifteen minutes from Terminals 2 and 3, a change if you are in Terminal 4. Walk to the train that takes you to the aircraft.
It is also on the Elizabeth line, so it is not a dead end if you go into the centre.
On the Elizabeth line, in the centre
Bond Street, Tottenham Court Road, Farringdon, Liverpool Street: pick the station nearest the one thing you are doing. The South Bank and the Tower are a short onward hop from there, not a reason to stay in a maze.
Check the walk from the station to the door with a suitcase. Ten minutes is the limit.