I believe the restrictions in place in the UK are a result of action taken by Honda UK to "protect" the established UK Honda franchised dealers from unofficial competition.
The fact that many of the parts, etc, had been discontinued by Honda, and as such were not available through the dealer network appeared not to matter. The fact that the dealer network and staff are generally more interested in the sale of new bikes, with little interest or knowledge in the older bikes appeared not to matter. Honda UK threatened, and here we find ourselves.
The fact that Honda have a bike in their museum with a reproduction seat on it will I suspect count for little here in the UK, given that the bike was restored in the US.
Good luck though Ash, I do hope you can get through to someone with a grain of common sense at Honda , and change the situation. After all, surely continued interest in the Honda brand resulting from the restorations we all aspire to has to be to Honda's global benefit?
Cheers - Chris R