... or even 55!
The rules are that you must retire at latest by 65. You can choose to retire with a pension at any time after 50 (rising to 55 in April 2010 - government legislation), albeit that the pension you retire on will be reduced.
Separately from that, you select what you wish to pay into your pension that determines its rate of accrual, but that doesn't commit you to any particular retirement age, if that makes sense.