Always the same, seems huge demand for cars from visitors to the UK (especially ex-pats like me returning home!) as well as Londoners renting a car to head out (also me in a previous life)
I try to book well in advance on a cancellable rate. In the past I used to get pretty good rates from Watford (can't remember which company ) - might be worth checking if taking a taxi out there or similar locations to collect would work out better value (watch out for opening hours though)