Prior to registering with a scheme I have always spoken to the local authority building control and stated that I would be undertaking the work and it would be under the building control notice, as
@lennyallover has stated this shouldn't incur any cost as it has been paid for in the building control charge, at that point they may want proof of competence (I used to give them a CV and qualification certs etc). In the past the LABC have sent their tester around to have a quick chat and ensure that I wasn't a complete numpty and knew what I was doing. Most DNO wouldn't care who you are or be worried about whether you are in a scheme prior to connection of a temporary supply. I used to just write out as much as I can get on a cert prior to connection for them. Is that what you mean?