I certainly agree that a 31 shouldn't be looped, and a -21 is much more useful. Could one have been? Possibly, it would've been untidy but I think do-able. TBH my mate's recollections are from 60 years ago, and its quite possible it was a sporting wing-over, chandelle or stall turn (yes I know these shouldn't have been done either, but it was a different time - see Pobjoy's story!)
Anyway, we digress. The point of my mate's story was how a week's gliding set a hook in him so deep, its still there. And I really think the RAF/ATC/VGS or someone has let a lot of kids down. IMHO of course