Technically, FSAVCs no longer exist, as they're just personal pensions now. All it means is if you use the in house AVC, then the extra contribution you're paying will be included with your standard pension contribution. Tax relief on that depends upon the basis operated by your employer.
If the main scheme is a final salary scheme then your contributions are deducted in full, and then you have tax deducted on the balance amount of income, meaning you get full tax relief at source. If it's on a money purchase basis, then they can either operate the same way, or on the net pay basis, meaning the deductions come out of your pay after everything else, and you pay an amount net of basic rate tax relief, so for every £80 deducted, £100 gets applied to the pension, and then anyone who's a higher rate tax payer has to apply to HMRC for the balance tax relief.
If you do your own thing with a Personal Pension, then that will be on the net pay basis as well.
The things to be aware of are that a PP is likely to have higher charges than the in-house AVC, but a PP will have a much wider range of funds available to invest in.
At any age from 55 onwards, you can decide to take benefits from the PP (or even cash it in in full - subject to tax!) as it's completely separate from the company scheme. If you've paid into the in-house AVC, then you can only retire early if the Trustees agree you can, and generally only if you also take benefits from the main scheme as well.
Also when actually taking benefits, the PP can be used to provide benefits from any provider. You haven't said who your employer is, but if it's a final salary pension, then often the in-house AVC has to be used to provide additional years pension. Which often can in fact provide a higher income than using the fund on the open market. Other schemes say you can use the fund elsewhere. It all depends what the scheme rules say.
Oh and yes, you're right, Friends Life (now part of Aviva) are shite.
