Claim money from unused health insurance (set by employer)

Unless there's something very out of the ordinary about your specific situation, private medical insurance policies in the UK, whether group policies through a company or personal ones, are just that: insurance.

You and/or your employer pays for cover, and if you need something that the policy covers, the insurance company pays for it. There's no refund available for "unused money" because you've received the insurance you paid for.