Your employer cannot normally force you to retire at a fixed retirement age. You might be able to claim age discrimination and unfair dismissal.
Exceptionally, an employer may set a fixed retirement age if they can show that this is a good way of achieving a reasonable objective. For example, a fixed retirement age might be used in occupations that need a high level of physical fitness. Even then, the employer would need to be able to show that the fixed retirement age was a reasonable solution (rather than individual fitness tests, for example).