You can get a valuation from
http://www.glass.co.uk/
but it's not free. The only values that matter for the trade are Glass and CAP Black Book (and you can't get a CAP valuation unless you are in the trade), not sure if the AA one will be the same, although it could be. These values are lower than the ones you can get from other places such as
http://www.parkers.co.uk/
Having said that I got an offer from a dealer which is around the Parkers valuation rather than Glass's valuation on the SLK55, so it's not an exact science.