I think it would be fairly easy to source buyiing a bike in the UK with an MOT (safety certificate) and road tax. Your problem will definately be with the insurance.
As I understand it, to have a British insurance policy on a vehicle you have to be the UK resident registered owner. As an example, I have a holiday home in France. My bike is registered in the UK to me and I have UK insurance on my bike which covers use in Europe. When I visit France, because my vehicle is road legal in the UK, it is road legal over there.
The rules in France are that because my bike is English registered, I have to have English insurance. There is an exception to this if I was applying to import the bike to France. I could obtain French insurance for a short period to cover the importation.
If I bought a bike in France, to obtain insurance on it I would have to be French resident, with the bike registered to my name and address, together with French insurance.
A UK insurance company would not insure the bike as it was registered abroad. If you enquire with a US insurance company, you may find they have similar conditions to insuring US registered bikes.
There is a possibility that if you knew someone in the UK who had a UK bike registered to them, they could put you on their insurance policy as a named driver. They would have to declare that you were non UK resident. This sort of thing happens when people swap homes for holidays between the US and UK and the swappers want to use each others cars while on holiday.
Your best bets are the above or hiring a bike, which will probably be more expensive.
Hope this helps. There's plenty of FZ owners over here spread all over the place, so if you get to the UK make sure you put a shout out on here first and maybe some of us could meet up. Bring plenty of beer money if you do !
Hope it works out. :tup: