I've used parkatmyhouse for parking in Central London and it's been great. Got a spot at a pub in just outside the congestion zone in Pimlico - £10.00 a day.
At Gatwick we use Maple Manor Parking who offer a really cheap and good pick up/drop off valet service. We are parking our car this time round for under £50 for the week - that's only a couple of quid more than the long stay. Thoroughly recommended.
also anybody flying from heathrow,i have just used discount code TDOWN on purple parking website, which halved the price,booked 11 days parking for £30, happy days