It's all in The Planning Portal ^^^ ........... but, if you're not quite clear, B/Regs are only interested if you have to make structural changes. Often, a Velux can be fitted by just taking off a section of roof covering, and fixing it in the opening. If there's anything like a purlin, brace, principal truss etc in the way, a Structural engineer would have to calculate the alterations, and involve B/Regs.
Thankfully, as a builder, I've usually found that a chat with the Inspector as to what needs altering, is enough for him.