Ginsters,
Assuming you were preordained to live until aged 80 and your annual AFPS pension is £10,000 at aged 60.
If you wanted to bring that 10k forward to aged 55, the state won't pay you an extra £50,000 (£10,000 over 25 instead of 20 years). So, it makes a reduction in your annual payment to reflect that. The people who determine the theory behind the reduction are actuaries and work in the Government Actuarial Department.
These are the same people who also work out how much you can draw each year from a personal pension (referred to as 'GAD rates') if you don't wish to annuitise your 'pot' immediately. GAD rates don't apply if you wish to 'plunder' your pot as quickly as possible for any number of reasons and are able to go into for instance, flexible (and not capped) drawdown.
There are downsides to doing that, and GAD rates are not etched in stone; there are dozens of types of risks which apply; anything from political risk to investment risk.
Public sector pension schemes
Income Drawdown
RPSM09103590 - Technical Pages: Member benefits: Drawdown pension: Flexible drawdown
Happy St George's Day everyone!