Parking at UK airports is getting more and more expensive with the arrival of low cost airlines - the airports have to make their money from somewhere car parking is one of the ways in which revenue can be increased - however pre book and you will save money and have priority at the car parks.
Look at these examples:

Manchester £ 26.95 8 days on site
Gatwick £ 49.50 8 days on site

Why not take the stress out of traveling and have a meet & greet service where a company representative will meet you at departures and take you’re to the car park and deliver it back when you arrive home – great service!

Manchester £ 59.50 8 days                          
Gatwick £ 79.95 8 days
Heathrow £ 126.50 2 weeks