When I grew up in the UK it was definitely celebrated but a very different holiday to the US. I don' recall treats but many tricks. It was not uncommon to wake next day with gates hanging from lamposts or worse.
What is so American are the treats and pumpkins and especially the kids out in their cute costumes (bumble bees seem to be very popular) collecting candy. I never saw that aspect of halloween in the UK. Most Brits I know in the US are not too keen on pumpkin pie, I think it's something you have t grow up with.