If after talking to your local building control authority, you find out that you do need building regulations approval, you can apply immediately to your local building control board.
If applying in person is not possible, you can visit the building control/ planning website and complete an online registration.
First find out what your project fee will be (if any) and how you can make payment. The process can typically be done in person there at your local building control office, via email, snail mail or even phone.
Your application will need to contain a copy of your new building plans for inspection.
On the other hand, you will not need to apply for building regulations approval if all you intend to do is install lights and power in an existing loft space as long as the electrical work is done by a qualified electrician who is registered with a competent person’s scheme such as NICEIC or NAPIT.