My thoughts
The K13 would have been the best in my view since it could be launched on the Eagle winch - had great visibility - was easily maintainable/repairable by the then MGSP organisation, coped well with short and rough fields - had no fatigue life - handled well, benign stall and spin and was affordable.
The L13 is a truely great sailplane but is weaker in all the areas listed above (apart from handling where IMHO it's superior to the K13) - and as you point out - why would we buy stuff from the then WARPAC countries ??
And the initial stimulus for purchase of GRP gliders for the Air Cadets was to soak up a budget underspend as I remember - at the end of a FY. Prior to that we just never seemed to have available cash at the right time...................
Right now I still think the Venture offered the best value to Air Cadet gliding in terms of number of launches versus number of trained cadets verus cost versus supportabilty versus effectiveness as an organisation (urgh
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