1. Renew your SEP Class Rating by passing a LST with a PPL SEP Examiner.
2. Add a TMG Class Rating by passing the TMG Class Rating Skill Test with a TMG Examiner.
Any hours flown on either Class will then count towards revalidation of both Class Ratings. Which means that all your Vigilant flying can then be used to keep both your Class Ratings valid, although you will still need to fly a 1 hr training flight with an instructor in the second 12 month period of the validity period to revalidate by experience.
It's all in LASORS!
Incidentally, there's no such thing as a 'Class A' or 'Group A' PPL anymore - and hasn't been for years.