The general benefit in kind rules apply if you take an medical insurance policy (which you benefit from personally) through your own limited company, the amounts paid will need to be reported on a P11d. You will pay tax on this personally and the company will be liable to a Class 1A National Insurance charge.
In the respect of life and critical illness cover, if you benefit personally from the policy this will be a benefit in kind also, as it is treated as a payment made on behalf of an employee.
However, if the company takes out the policy, the premiums on such a policy will be allowable if the sole purpose of taking out the insurance is to fill a hole in profits that may result from loss of the services of the key person. The only downside is that any proceeds by the company are taxable as trading income. There are a few execeptions when the policy relates to a director of company, and we would recommend you speak with your local TaxAssist accountant to discuss these.