Under Permitted Development (PD) rules, Part 1, Classes B and C, you are able to make certain additions or alterations to roofs. I suggest that you appoint an architectural designer who will be able to advise whether your roof replacement project falls under PD rules. If not, a householder planning application will need to be submitted.
Building Control approval will be required; your designer will submit drawings to Building Control. You will need to appoint a structural engineer who will produce loading calculations for the roof structure.