Since British power went into decline there have been anxieties about growing American influence in Britain (and elsewhere in Europe for that matter). Often these take the form of concerns around the perceived import of an ever increasingly crass commercialism and invented traditions that have more to do with marketing than any deeper roots.
It’s not xenophobic per se to take issue with these things.
As for Halloween, for anyone over, say, 50, it was never a big thing in Britain growing up. You got the odd trick or treater who was typically regarded as a pest, and might see a few youngsters with a half-arsed costume on in a pub. Celebrating Halloween was often dismissed as an “American thing” and, as such, seen as lacking authenticity. Bonfire Night was a much bigger occasion, although sadly it seems to have been in decline for years.
American cultural imperialism is real, and while the country exports some great cultural product, not everyone, rightly, is happy just accepting all aspects of it. They see it sometimes as just another money making exercise, or a ritual that seems unnecessary, or a lesser version of what we already have which the import then crowds out. You may not agree, but I wouldn’t call it xenophobic. In many cases it’s entirely reasonable.
Also, as others have pointed out, perception of America waxes and wanes in Britain elsewhere. We’ve had years now of the idiocy of Trump, police violence, ICE, the Iran conflict, fools like RFK, peddlers of damaging social media, Musk egging on fascists here in the UK and so on. I’m sure most Brits know this isn’t all Americans, but it does influence perception. When the Trumpian tide eventually turns opinions will too.