The APD is exempt if you are in transit. For long haul this is GBP 40 at the back and GBP 80 up front.
You are still required to pay the UB ( Passenger facility charge ) which is GBP 21.20.

At United, taxes and service charge - if applicable, is deducted from your salary after you travel.
As a UK based employee an exemption would not be applicable, but I think it would be difficult to provide tangible proof that you are in transit,
especially if you were standby into the UK. Not saying it's impossible, but we're not talking a few bucks here, so potentially rewarding !

In the UK we have tried many different variations to avoid the high tax, especially at the front of the aircraft, but factor in a ZED in coach
within Europe we still need to pay the GBP 21.20 and GBP 10.00 APD, add the ZED fare, then the departure taxes for FRA, CDG, FCO, AMS, BRU etc ....
less flights = less opportunity to boot, what a pain !
Don't get on = hotel accomodation ... Europe ain't cheap.

They've got us by the short and curlies