I guess it varies by region of sales. I'm pretty sure demo bikes have to be registered to the dealership with insurance requirements and the warranty clock starts at that time. At least in my experience, then they'll try to sell you an extended warranty to keep it going. They were throwing crazy deals on demo bikes, but you know how those are ridden. Brand new bikes will still have the UK registration unaltered by title transfer.
I'd either look at a low mileage owner for sale or a brand new (no demo) bike. There are some new '21 '22 ... ones that weren't demos, you can tell if it has more miles than just a standard checkout ride. The price is certainly negotiable since the Storms have came out. It just depends on what you want. I'd personally never buy a demo bike, whatever the warranty.