Sickness absence review
A. Probably not. Unlike your right to sick pay, a sickness absence review procedure is unlikely to be a term of your employment contract.
If it is not a contract term, this means it can be changed without your formal consent. If a union is recognised at your workplace, there should be collective consultation before any change is made.
In particular, regulation 4a of the Safety Representatives and Safety Committees Regulations requires employers to consult with safety reps about any workplace measure which may substantially affect health and safety.
This would include a plan to significantly overhaul the sickness absence review policy.