As you approach retirement age, one of your retirement options may be to purchase an annuity. This is initiated by the pension company that have been managing your pension fund.
You should be offered an annuity rate from this company and also be made aware that you are free to purchase your annuity elsewhere.
This option is referred to as an 'Open Market Option'. This means that you can move your retirement fund to the company that will offer you the best annuity rate available in the open market.
This type of Annuity is also referred to as a 'Compulsory Purchase Annuity' ie where the money being used to purchase the annuity/pension is coming from a pension fund.