I am guessing here but, the 14 day cooling off period is probably if you cancel it before the policy starts, not 3 days after it starts.
I support an insurance company in that a lot of the cost of a policy (excluding claims) is at inception, and therefore if you cancel, they should be able to recover reasonable expenses. Whether the insurance company and the broker have both incurred £25 of expense is questionable, and therefore the agreement to refund half of it is reasonable in my opinion.
Dave