I'm sure the all season tyres are very good, i've read many a review praising them. But i'm not sure I would want to incur the outlay and the hassle of a second set of wheels if I wasn't going to get the benefit of a full winter tyre. The difference between my Pilot Alpin 4's and the Bridgestone's is night and day in winter driving conditions. Winter tyres aren't just for snow, the softer rubber compound sets them apart from all season tyres. Anything sub 7°C is ideal for the winters, which for me is about 5 to 6 months of the year for the time of day when i'm traveling to and from work.
If the all season tyres are good enough for winter use, then surely they're good enough for summer use too? They exist to give you better performance than a summer tyre in winter, and better performance than a winter tyre in summer. If that's all you want then all season tyres will be fine. But they won't match either the summer or the winter tyre in the respective seasons they were designed for.
I fully agree with all of the points you’ve raised. Perhaps, I was not clear in my point of view, so let me clarify.
Winter tyres are better than all season in winter, I am not trying to suggest otherwise. Most summer tyres are better than the CC+ in summer. This last point is my personal view (not scientific), I preferred the feel of the CC+ compared to the S001 when I replaced them last autumn, but Ive no doubt the S001 would be quicker against the stopwatch in warm and dry conditions.
My point is that in most driving conditions during UK winter months the CC+ would be as good as a full winter, the difference would be minor in the 5 degree wet road conditions you mentioned but the trade off is they have the dynamics of a summer tyre so will mostly likely feel nicer when driving, which is a key factor when you own a high performance golf. In more extreme climates in the UK, I would recommend winters every time but the Sunderland area does not have an extreme climate.
I am of course biased as I have two sets of wheels, one with CC+ tyres and the other with CUP 2s.