Their website claims they can produce 70-80 or so gliders in a year
That is of all types, not just K21s. The others are ASH26, ASW28, ASG29, ASH30, ASH31 & ASH32. I didn't realise the ASH25 had been dropped.
Also suspect that if the RAF / ATC take it into their heads to modify any they buy I can see AS washing their hands of them.