AVCs not good for me!
Firstly, please get professional advice!
My own experience was not good. I started AVCs in early 90s and left the Army after full service (pensionable) of 22 years in 99.
Despite an Army / MOD pamphlet that clearly stated I had pension 'headroom' due to flying pay and higher than average pay band, the AVC provider returned all my premiums in 99 stating that I had no such pension headroom.
Besides this, one thing I hadn't realised was that the AVC (yes they were freestanding) had to be taken at the same time as the pension was first payable.
Whilst it seemed I had the grounds to fight my case, the return would have been so poor I didn't bother and reinvested into something else.