The Vigilant T1 does not meet the requirements of CS.22 for a motorglider, therefore legally it is a light aeroplane. Specifically the maximum permitted MAUW for a motorglider (as shown in CS.22) is 850kg, whilst the Vigilant has an MAUW of 908kg.
However, around 10 years ago, CAA PLD ceased to accept this and chose to regard it as a motorglider. So far as I know, they haven't changed this position. In my opinion this is not a supportable position on their part, and I did provide the written proof for a couple of people some years ago, but they've not changed it. You could always try again.
Flying within the Air Cadets organisation will be subject I think to their own training and qualications, and your civil qualifications are likely only to be taken as proof of experience.
G
(In a previous life, the engineer at Boscombe Down who wrote most of the Vigilant certification reports.)