Aldo is correct, the UK has the highest number of whirlwinds and tornados on the planet, it's just that most of them are pretty localised...
http://www.torro.org.uk/TORRO/php/gallery.php?category_path=tornados&start=0
It's just surprising to see one of such strength.
I don't know, I really don't see any damage greater than a strong F0, allthough the possibility is there of it being a very weak F1 when it pulled the roof off that house. IMO, the tornado that hit the UK last year was much more impressive, I think it was a strong F1, and had one of comparable strength hit this area, we would be looking at a much more grim aftermath.
EDIT; highest frequency of reported tornados, at least. Although apparently there's a suggestion Holland could now be higher. Plus, the strength is lower (in the UK) and you wouldn't see the monster F4s and F5 in the US.
Quite true, the reason is that the UK does not see as many "supercell" type storms, most of the UK tornadoes are ether cold core, or waterspouts that have moved inland, allthough every now and then you will get a mesocyclonic storm like that second and third pics of the link Flip posted.