You should always check the exclusions before taking out any insurance policy to ensure that you understand and are comfortable with them. In general, this type of policy will not pay out if you stop paying your premiums, are diagnosed or treated for a condition not covered by the policy or that does not meet the insurer's definition of a critical illness, or pass away pre-diagnosis (if you have a combined life insurance and critical illness policy then you would be covered by the death element in this particular instance).
Some policies exclude certain types of cancer and others state that you have to suffer from permanent symptoms in order to claim for some conditions.
This is an insurance product which can become very complicated and create a lot of heartache and stress should a claim be rejected. Therefore, it is wise to discuss your requirements in detail with your policy provider before signing on the dotted line or seeking guidance from the Citizen's Advice [4] team before taking out a policy, especially if you are doing this to guard against a lack of savings or concern over future financial planning.