Try looking at Practical Classics on line edition, they have lists of Classic car value guides there.
http://www.practicalclassics.co.uk/should the club keep a history of sold coupe prices to assist owners with market values ?
Anyone can look through the coupes for sale section and see what prices have been asked, but that will not necessarily show what has been achieved. "The Club", as you put it, has no further information than that. Nor does it have the people to do so.
It is run by a very small group of volunteers who do have other things to do in their lives. Requests for other people to volunteer and help are almost always ignored.