Preferably at the time you make your application. This will entitle you to interim cover while your full application is being processed. The payment is fully refundable to you if you elect not to proceed with the insurance. However you can arrange for the first premiums to be paid from the Direct Debit Authority which you file with your application and, from which, all future premium payments are made. It is very important to check the official claims paying rating of the insurance company - this information is provided for you in the company comparison section of this site.
Some companies refuse to allow themselves to be rated.
InsuranceLink provides products from only companies which are rated and have either an Excellent or Superior rating. Smoker rates are higher than for non-smokers. Once the policy is issued it cannot be cancelled by the insurance company providing that the required premiums continue to be paid. For a small additional premium the insurance company can agree to waive premium payments if you become disabled. It is important to check each companie's definition of 'disabled' to ensure that you know what protection you are obtaining for the additional premium.