someone else can explain better than me, but.. humidity is lower when it's cold but relative humidity is higher. Being outside in the uk when it's cold is often like being inside a cloud, with 100% relative humidity levels. Combine this with the old, badly insulated, difficult to heat housing stock, and you can guarantee that there is one part of the house (a north facing wall, say) that is at or dangerously close to the dew point for some part of the day/night. You wouldn't want to leave a steaming bath for longer than necessary.

The UK climate has moderate temperatures and a lot of rain. Excess humidity is a common problem here, and excessive dryness is rare.

It tends to often be humid here, even in winter.

Right now, it’s 81% humidity. Our laundry has been taking two days to air dry.

Cold air can hold much less water (hence why it's relative humidity). This is good if you take in cold air and heat it to 21°C or so because the air you end up with will have very low humidity, but if the outside is a drizzly 10°C and you are only heating to some 18°C, it can be a problem.