No problem, you have 14 day cooling off period under FSA rules. So just phone them to cancel and then start the other policy.

I took out insurance yesterday which is due to begin on the 1st sept. This was with swinton insurance including 50 cashback. After a conversation with a friend I ran a quote adding my boyfriend onto the policy, it came back £100 cheaper with £75 cashback but still with swinton. Am I too late to cancal the original policy and then take out the second?