I have done this a few times.
HeathrowWhat is "best" depends on where you are starting from...
- if you're starting from west London, go to Paddington, see someone there, then take the Heathrow Express
- if you're east / southeast, try someone on the Piccadilly Line (Gloucester Road-ish, where there are plenty of WGs). Then get back on the Pciadilly line and go to Heathrow. Don't bother with the express, by the time you get to it you only save about 15 mins
At Heathrow, the hotels in Bath Road etc are actually very slow to get to / from the terminal. Shuttle buses are slow and taxis few and far between. Plus quality of WGs not always the best in my experience.
Solution - the hotel at Terminal 5 does day rates - book that, check in at T5, get rid of the bag, walk to the hotel and see the girl of your choice then walk back to the terminal and go through security. I have done a few Reverse bookings this way and it worked out a lot better than booking girls at their outcall rates
CityBetter to see someone Canary Wharf-ish then hop on the DLR or take a cab to the airport.