For much of the UK winter the outside relative humidity is near 100%, ...
That's a bit high. Here's the averages for January, generally the coldest month:
In recent years I'd say 70% RH, based on my own outdoor readings, was a more typical figure for my little corner of the world (urban East London).
The chart says much of the UK is in the 86-88% range. Over large areas the RH certainly exceeds 100% night after night for much of the winter, because there is frost or dew on the ground each morning. If its that high at night and 86% on average, that suggests a typical midday figure might only be 70% or so. That seems lower than measurements I made in North London/South Hertfordshire in the 80s. Typically, 40-70% RH is considered the comfortable range.
Winter is certainly characterised by a pretty high exterior RH over much of the country, but heated buildings can drop that figure so much that people complain of the dryness, and wooden furniture often warps.