This is sometimes confusingly called ‘mortgage payment protection insurance’ or ‘PPI’ but is different from ‘mortgage protection insurance’ as above. It’s cover would keep up your repayments for a time if you are unable to work because of illness, accident or being made redundant. You need to check carefully if the policy is suitable because many will not cover self-employed, part-time or contract workers, for example.
ASU policies are quite expensive. Also, most will not pay out until a few months after you are unable to work, and then for no longer than a year or two. So whether you need this kind of cover may depend on whether you have enough savings or other assets that could keep you going for a while.
The Money Advice Service has produced a useful guide to whether or not you may need PPI that can be found here.