I highly recommend Priority Pass. It provides a great deal of value to the frequent flyer.

Frankfurt Main (FRA)
Europe City Club (ECC) - Terminal One, Departure Hall B (landside)
Delta Crown Room Club - Terminal Two, Concourse D
Sky Club Lounge - Terminal Two, Concourse E

London Gatwick (LGW)
Continental Presidents Club - South Terminal
Sir Robert Hardingham Suite - South Terminal
Servisair Lounge - South Terminal
Alexandra Suite - North Terminal
Delta Crown Room Club - North Terminal

London Heathrow (LHR)
Business Centres, Heathrow - Queen's Building (landside)
Servisair Lounge - Terminal One
Servisair Lounge - Terminal Two
Servisair Lounge - Terminal Three
Kuwait Airways Oasis Lounge - Terminal Three
KLM Holideck Lounge - Terminal Four

London Luton (LTN)
Executive Lounge

London Stansted (STN)
Servisair Lounge - Satellite One
all locations airside except where noted