This is insurance that may pay the hospital or doctor for eligible treatment, subject to the policy terms, limits and exclusions. It can include treatment in a private ward or getting seen earlier in an NHS ward. Some plans allow you to claim if you are not able to be seen by the NHS within a set period. Other plans may charge a little more and don't have any relevance to NHS waiting times.
Underwriting requirements vary by insurer and policy. Pre-existing conditions may be excluded, and you should check what treatment is covered before taking out a policy. Premiums are usually reviewed annually.
The plan will have no cash in value at any time and will cease at the end of the term. If premiums are not maintained, then cover will lapse.