.. The problem is that global warming has
increased the temperature of the North Sea, so the easterlies are being
warmed up sufficiently that the temperature is hovering just above zero,
rather than dropping below zero as it used to.

Don't agree with this theory because they are now at 6C on our coast and
only 2C on the other side by Norway. What would they have been in an old
fasioned early January? No lower than this and we still got cold dry
easterly's in January.
IMHO it's because the air has been very moist and not that cold to start
with and the soil, in the south anyway, is well above freezing and warmer
than the air above. Surfaces like cars and dustbin lids (plastic) have
frozen water droplets but the grass is unusually uncrunchy and just wet.

Dave