Until 1999 there was no choice in the matter of when you took your FSAVC: Once you began drawing benefits from your main pension scheme, you also had to take the benefits from an FSAVC plan.
In summer 1999 the Pension Schemes Office (PSO)Update No 54 changed all that. This stated that AVC and FSAVC benefits may now start at any age between 50 and 75, regardless of whether the member retires or leaves pensionable service.
Unfortunately the decision to implement this is entirely voluntary so insurance companies can simply refuse to offer this facility. And as offering it involves altering the trust deed and the rules of their plans, not all of the FSAVC providers have chosen to take this step.
Hope this is of some help and good luck.