Hi,
It all depends on your personal circumstances: Married or single? Dependents? Profession? and your attitude to risk?
If you are single with no dependents then critical illness on it's own is usually advised.
If you have dependents then critical illness + life is usually advised.
Decreasing term is normally associated with a repayment mortgage. level term is normally associated with an interest only mortgage or someone who wishes to retain a specific amount of cover for the term insured.
If you are single with no dependents and you are concerned about retaining an appropriate amount of cover if you are unable to work, Permanent Health Insurance is a viable option.
As with all insurances an adviser will recommend the most appropriate level of cover for your circumstances, but at the end of the day it is the your choice. The longer you leave it the higher the premiums will be.
Regards
Ian