Lawyers, bless 'em, are way out of their depth with these kind of technicalities. They simply insist on covering themselves. Lack of Building Regs Approval can come back to bite solicitors.
As others have said, B Regs are not interested in re-covering roofs. Material alterations such as the introduction of breathable felt and/or roof ventilation technically can be classed as notifiable. In practice, this is taken as obvious, and I can't think of any re-roofing in my experience, that has had B Regs involved.
The archway could be relevant if it's a structural wall. A retrospective application is one way to go. Should be a formality after 25 years!
This kind of thing has to stop. The Indemnity Insurance Companies are onto a little goldmine with these. They love them since there is little or no risk involved for them.