First up, winter tyres make sense anywhere in the UK. The amount of snow you get makes sod all difference. Summer tyres stop working below 8 degrees and don't perform particularly well in the wet either. Winter tyres are perfect for the wet and greasy cold(ish) conditions we get in the UK throughout the winter! Given you don't wear out your summer tyres while the winter tyres are fitted, other than the initial investment, there's nothing to justify! The safety aspect of reduced stopping distance in the wet is all the justification you need IMHO!
As for the best size to fit, that's really a question of what performance you want from the tyres and how hard the winter conditions are. I've always found the standard wheel/tyre size for the Scooby is perfect for winter tyres with 1" greater diameter wheels and 1cm wider tyre width for the summer. So winter tyres on classics 205/16, New age 215/17, Hatch 235/17. In milder parts of the UK, you could probably get away with larger sizes, particularly if you don't go for 'V' speed rated tyres.
As for the best tyre, that's always subjective like with summer tyres. Although like with the tyre size, there are trade off's you can make with the choice of tyres. If you go for a performance (V rated) tyre, then you will sacrifice some of the snow/wet performance, although you may gain on ice performance and you won't be speed restricted. Go for a lower rated tyre and you'll have great snow and wet and ice performance, although you're top speed will be limited by the tyre rating and you'll lose performance and have higher tyre wear in milder temperatures (>15 deg).
Many people here have fitted Nokian's in the past, although I've never tried them myself. Personally I prefer more performance oriented tyres so I can still go fast on the Autobahn when I need to and generally the roads are clear of snow for most of the winter. My first set of winter tyres were Vredstiens on the RB5 and they had great snow performance, but speed limited. On the New age I had several sets; I wasn't impressed with Michelin Pilot Alpin (PA2), they were pretty much undriveable in the snow (although I hear the newer ones are better), I was much more impressed with Dunlop Wintersports and Goodyear ultragrips, couldn't really say which was better.
On the hatch I've been running Pirelli Sottozeros with 235 profile and have to say these are the most amazing winter tyres I've ever used. I was initially really worried about running 235 winter tyres; I once tried a set of 225's on the blobeye and they were not so great, so I was sure 235 would be a lot worse in the snow, but actually, the snow performance has been as good as any performance tyre I've used in the past with much smaller widths and generally outperforms the other tyres in all other conditions. They're also incredibly quiet for winter tyres too! I've been so impressed by them, I just fitted a new set
I also have a set of Autosock snow socks, which I carry round in the boot the whole winter, although I've no idea how good they work, I've never had to take them out of the bag Winter tyres perform so well with AWD, chains/socks pretty much become redundant. The only reason I carry them is because its a legal requirement to carry chains on many of the roads up to the ski resorts round here - Its a bit like the spare bulbs, first aid kit and warning triangle that sits in my boot doing nothing Maybe one day I'll get to use them and can give a proper review. I certainly wouldn't consider them as an alternative to winter tyres though, they may work well for the one day you need them, but winter tyres will work well every day you have them fitted!