There was a thread some time ago concerning the wearing of miniatures at formal functions by those in civilian dress. The answer seems to be when military personnel are present in mess kit.
At Old Cranwellian Reunion dinners, the dress for all, serving and retired, is black tie, no medals or neck decorations. At Test Pilot Dinners at Boscombe, where serving and retired guests are present, dress is mess kit or black tie, miniatures to be worn.