For Heathrow, crews can be informed by adjacent FIR's if the delay is over 5 minutes and the crew can sometimes be told to reduce speed to absorb some of that delay. London ACC will also instruct the aircraft to descend at 250kts when possible to absorb some more. Delays at any other UK airfield will be passed to inbound traffic by ATC as and when the message is passed along the line to slow them up.
EATS will be issued for Heathrow and Gatwick when the average delay is 20 minutes or above, and Onward Clearance Times should also be passed if holding occurs at an outer "enroute" hold.