What is the temperature outside? Humidity is measured relatively (to how much water air can hold at the current temperature), so even 100% humidity outside at a moderately chilly temperature outside can still translate into uncomfortably dry (e.g., less than 35%) humidity inside.

Currently about 17c in the day, 14c at night. At the time of writing, outdoor is 13c and RH is 95%. It was 22c the other day, however it is quite unseasonably warm right now. Once it gets into proper winter weather, normal daytime climate is around 10c-ish and 80%-90% humidity. I've never seen my indoor humidity lower than about 50% without a dehumidifier except after a really long dry spell in the summer or a really cold day in winter.

It doesn't often get that cold here, just very damp. Global warming is making winters even wetter. The UK actually has a fair bit of "temperate rainforest" and is classed as a humid temperate oceanic climate.

What region of the UK are you located in?

Inner London, which is going through a bit of a damp period right now after a really dry summer. I'm not sure if those two things are related, but technically we are still in drought even though my garden is basically a swamp right now.

My tropical plants are enjoying it though.