As a starting point you can get an immediate quote from several online sites (webuyanycar, wewantanycar etc) just by typing in your reg # and giving a brief description. They don't seem to give much value for optional extras which certainly help if you decide to sell privately.
Are you suggesting WBAC are massively undervaluing to ensure they make a huge profit out of unsuspecting punters under the guise of it being a 'safe and easy' transaction?