You can normally buy an annuity as soon as you are 55 – you do not have to have retired in order to be able to do so. There used to be a rule that you had to buy one by the time you are 75, but this regulation has been abolished. However, your own scheme may still have an 'age 75' rule.
If you have several different pensions, you do not have to use them all to buy an annuity at the same time. Indeed, many personal pensions are deliberately arranged in a number of separate pension policies so you can phase your retirement.
However, this is probably only useful if you have a substantial amount saved. If you have a number of fairly small pension pots, you may need to combine them before you can buy an annuity. You should take financial advice before you do this, and remember that if you draw your pension before your normal retirement age, there may be financial penalties.