Flexible retirement is where you reduce either the hours you work or move to a post from which you receive less pay and receive payment of your pension (in part or full), as well as your salary.
Once you have reached your 55th birthday, you can ask your employer if you can be considered for flexible retirement.
It is entirely up to your employer whether they let you retire on these grounds, with any applications being treated in accordance with their policy statement.
If your employer grants you flexible retirement and you draw your benefits before your 65th birthday, then all, or some of your pension benefits may be subject to a reduction.
It will largely depend on whether you meet the 85 year rule.
Your employer has the power to waive any reduction if they wish, on the grounds of compassion, again, details of this can be found in your employer's policy statement.