There is no requirement for schemes to have ill-health retirement provisions and therefore no minimum standards exist. If you are in a 'money purchase' (DC) scheme, you may be able to draw your pension early but there will probably not be any special provisions.
Your employer may instead provide an income replacement scheme – sometimes called a PHI (permanent health insurance) arrangement.
If you are in a salary-related scheme, the terms you get will depend on the scheme rules. Some very good defined benefit (DB) schemes will pay you the pension you would have got had you worked through to the normal retirement age if you have to retire early because of poor health. Other schemes may not be quite so generous, but still assume you have worked extra years.
How much pension you get may also depend on whether you have been assessed by the doctors as unfit for any work in future, or only unfit for your own job.