In reply to Rob Exile Ward:
One thing to bear in mind is that while summer tyres may be legal up to the point where the conditions mean that chains/socks become necessary, once fitted, they still put you at a disadvantage relative to “properly shod” locals.
This point came across to us very clearly a few years back while driving on a Swiss motorway after dark in a snow storm with it settling rapidly on the carriageway despite the relatively heavy traffic in both lanes. The locals were all motoring along nicely on their winter tyres at 30-40 mph. We had snowflake rated 4 season tyres which coped really well and gave an increasing sense of confidence when it came to keeping up and braking. Summer tyres would have been far too unpredictable in those conditions and stopping to fit chains/ socks would have been horrendous.