This looks very iffy.
- It purports to require you to consent unconditionally to medical examination.
- It further purports to demand unredacted medical examination irrespective of relevance.
According to http://www.acas.org.uk/media/pdf/n/9/B11_1.pdf
The Access to Medical Reports Act 1988 requires an employer to obtain written consent from a worker before applying to his or her doctor for a medical report. The Act lays down a procedure to be followed and gives workers the right to see the report, to request amendments or to withhold consent to the report being supplied.
The relevant provision is available at https://www.legislation.gov.uk/ukpga/1988/28/section/3
While there may be theoretical loopholes to this, an employer would be unwise not to follow the norms laid out above.
It would be ethically questionable for a doctor to examine without consent (where a person has that capacity); or to rely on a contract entered into some time ago under different circumstances as evidence for consent.
In practice an Occupational Health professional will almost certainly want to protect themselves professionally by establishing genuine informed consent first.
The TUC has more guidance on the complexities of this:
The contract term may also be unenforceable under GDPR insofar as it is a) processing data reliant on consent which is not freely given and b) processing more sensitive data than is strictly necessary for the task.
Now just because you have the right to refuse consent doesn't mean the employer will do nothing about it.
The employer might attempt to refuse payment under an occupational sick pay scheme, to which some reasonable additional conditions may be attached, but a) so long as you notify them and get appropriate doctor's notes in the usual way (see https://www.gov.uk/statutory-sick-pay for details) your employment contract cannot invent additional conditions to prevent you receiving Statutory Sick Pay, and b) it is questionable whether submitting to a disclosure arrangement as described would be reasonable.
The employer might attempt to discipline or dismiss you for refusing to comply. The reasonableness of this is likely to depend on the existence of exceptional circumstances. There may be cases where there is a statutory duty to monitor for specific exposures which give rise to a refusal to consent being a sufficient reason for dismissal (and I do not know for certain) although the contract wording given does not lead me to expect that this is the case here.
In the absence of such a reason, any detriment might lay them open to a claim of victimisation of an employee asserting their rights. If the employer takes any such detrimental action, you would likely need to submit a grievance and be willing to pursue to Tribunal to sort it out.
If you are currently being required to submit to an examination under this term, seek advice without delay from your union representative. If you did not have the prior presence of mind to join a union, a solicitor specialising in employment law may be able to give advice (for which you should expect to pay).
Having said all that, I notice the term does not specify that the employer may insist on who does the medical examination... I do wonder if you could meet this requirement by simply visiting your own GP, who will presumably be bound by the legislation above (definitely get advice before trying to use this!).