UK Airport Parking 
Airport Parking Discounts are a UK based service offering cheaper airport car parking than you the standard turn-up rates, which are the rates you would pay if you turned up at the airport unannounced. As we write, the site offers discount for over 120 UK airports including Heathrow and Gatwick in London and of course Birmingham, Bristol, Luton, Manchester and Glasgow as well as others. You have a choice of Off or On-Site airport parking, Long Term, Short Stay or Meet and Greet parking.
http://www.airport-parking-discounts.co.uk/

Heathrow Parking 
Get the best deals on Heathrow parking as well as exclusive Heathrow Hotels & Parking packages from UKs original airport parking comparison site! Instant savings, secure online booking and honest customer reviews.
http://www.airport-parking-quote.co.uk/parking/LHR121-Heathrow-Airport.htm

Gatwick Airport Car Parking 
Official Gatwick Airport Parking - Park closest to airport terminal. Reserve your space online and save. No-one sells our parking for less. Our recommended airport parking at Gatwick is of a high standard, offering secure, well lit, manned car parks which are clearly signposted from all major roads approaching the airport.
http://www.gatwickairport.com/parking/