You could try wewillbuyyourcar.com. This is different to the previously mentioned webuyanycar.com.
I used the former earlier this year when I had to get rid of my S quickly - got a good deal on a Works and could not face driving my old car to London to trade in. I valued accurately on their web site they inspected the car 2-days later, confirmed the web site valuation and did a CHAPS transfer. Got the trade price which suited me as I was in a hurray. I did try the webuyanycar.com but was not impressed, they do not do the instant CHAPS payment.
Cheers
Neil