I may be missing something obvious here but I don't understand why UK IMCR holders should be required to pass an IR checkride to continue exercising their IFR privileges in the UK.
If they wanted to exercise them in say Germany then it would make sense. In Germany you play by German rules, etc.
Anyway, the vast majority of UK flight on IMCR privileges is going to be OCAS and thus falling outside of the remit of any regulatory authority to dictate the individual's attitute to risk.
I don't know if I would pass a JAA IR checkride today; probably not... depends on how much raw data single-nav-instrument stuff is involved.