It sounds like this is a reviewable policy. Usually the company has the right to review it at any time but usually the first scheduled review is 10 years after the commencement and every 5 years thereafter (annually after age 70). You need to check the policy conditions to see if this is the case. The options you set out above are the typical options that come out of such a review.
If you are in good health you could see if there are cheaper options in the market for same cover for the remaining term. You will have to pay a higher premium than the original premium of 437 but it may be cheaper than 875.
It is worth mentioning that reviewable policies are still a major source of complaints that go before the Financial Ombudsman (eg policy was sold as guaranteed cover, reviews being carried out late). If you still have the paperwork, you could see what was promised to you at outside.