Curious to see the differences across the countries.
I’d say far less than half based on my experience - I’ve very rarely seen people putting out a pumpkin or other Halloween decorations in their houses.
The one aspect of Halloween celebrations that I think does exist in the UK is trick-or-treating, which I did do as a kid, but even then it’s been probably over a decade since I last had a trick-or-treater knock on my door, though that may be partly a London factor.