One reason for that could well be that many dealers wouldn`t care to list some un- or semiknown 10 to 20 UKP records as they assume there is no demand for them. There is a vast number of tunes that you indeed very rarely see up for sale anywhere. When they have become "envogue" you then suddenly seem to find them anywhere. Recent example springs to mind in the shape of Chyvonne Scott on Alto...I was after that for years, found one, played it out, years went by and the record has somehow become "indemand", sells now for 100 UKP and you can get it nearly everywhere.
Marc