From Richard's link.

In assessing the reasonableness of the force used, prosecutors should ask two questions:

was the use of force necessary in the circumstances, i.e. Was there a need for any force at all? and
was the force used reasonable in the circumstances?
The courts have indicated that both questions are to answered on the basis of the facts as the accused honestly believed them to be (R v Williams (G) 78 Cr App R 276), (R. v Oatbridge, 94 Cr App R 367).

To that extent it is a subjective test. There is, however, an objective element to the test. The jury must then go on to ask themselves whether, on the basis of the facts as the accused believed them to be, a reasonable person would regard the force used as reasonable or excessive.