This is known as 'disability leave' and is classed as a reasonable adjustment under the Equality Act (see the FAQ on reasonable adjustments). It should be recorded separately to sick leave. Your Force must consider granting reasonable requests for disability leave - note that this includes recuperative therapies and medical check-ups if you're otherwise well and able to work.
Unison have prepared a factsheet on disability leave - this is intended for union reps, but contains useful information for anyone needing to request disability leave for themselves.