Whether you switch from summer tyres (the Primacy 3 tyres) to Cross Climates (all-season tyres) is personal preference, but given the adverse weather conditions the UK often faces it may be worthwhile. All season tyres have slightly less dry grip if you're an enthusiastic driver, but they are better all-round in wet and cold conditions.

As for running CrossClimates on one axle: it's not ideally recommended for long-term use because the tread profiles will be completely different, which could have an effect on wet handling or braking.