You can normally buy an annuity as soon as you are 55 and must buy one by the time you are 75 (although your own scheme may have stricter rules). You can buy an annuity when you are still working.
If you have different pensions then 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.
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