Don't go it alone! You may find that the company are willing to make their 12% contribution to a private pension scheme of your choice - it's worth asking, especially if you're not happy with BA's choice of pension scheme.
As for AVCs, you can top up pension contributions via AVCs up to the limit of income as determined by the Inland Revenue. For a personal pension, this limit is age-related. For a company (i.e. group) scheme, this is 30% of salary (total of pension and AVC contributions, including company contributions). See the Inland Revenue's website for more info - alternatively, get hold of a copy of Intuit's TaxCalc which has a very useful guide to pension contributions. I'm sure Microsoft Money has something similar. If you're in BALPA, have a chat with their financial people.