Like many of the 32 UK species, these over-winter as adults. Also seen this month has been the larger Green shieldbug Palomena prasina, except in winter this turns brown. The coloration will green up next spring, as the shieldbugs start to feed on their host trees once again.
These two species are commonly found in urban areas, in parks and gardens, sometimes seen on ivy blossom. Maybe they were in town shopping around for late nectar or winter accommodation.
Some more information is at: