Scott C
I think existing VGS instructors were able to add a TMG rating to their licences based on their air cadet time. However, this was allowed, I believe, as they had many hours on the Vigilant.
You really need to ask the CAA as it's their opinion that counts in the end. A friend of mine who is airline examiner mentioned that this may change under EASA rules anyway. If you look in LASORS I believe that TMG time, if you hold the rating, will count towards the experience for your biannual.