Webuyanycar will give you a reasonable offer, but not the best you could achieve with effort.
However their staff are trained to reduce the eventual agreed price once they see the car. They receive commission on that reduction. There is then an 'admin fee' added.
I have to say though that the process is very easy, and possibly the reduced amount you receive is worth it for the ease of the sale. Worth standing up for yourself and being prepared to walk away to get the best price from them.
A higher price would probably be achieved using motorway.co.uk, though more effort is needed.