It's a european model, I'd guess. Except for the smaller turn sigs it is like a 76 US model 550F with the accordian boot forks and the 2 stay front fender. The black tail light mount is the first I've seen, US models were chrome. Maybe to accommodate the billboard license plate? The color looks right for a US 76 model, too.
The handle bar end shouldn't be open like that. Probably an incomplete bar grip change.
Looks like a pretty nice example in the pics. In the states, that price would be top dollar "for collectors only". Of course, it wouldn't be period correct for a US model.
I don't know how to value a Brit model bike.
They are very nice bikes, though, I've got 4 of them.
Cheers,