I wasn't aware that Halloween was an "American" holiday. My English friend has had a Halloween fancy dress for the past 2 years.
Gretel, I think it depends on who you ask! Halloween does have its roots in the traditional Celtic festival of Samhain and the Christian holiday All Saints Day.
However, some Brits tend to complain that the OTT commercialised version as celebrated in the US is 'taking over' here in the UK now, so in that sense Halloween is perceived (now) as an 'American' holiday.
Over here, it's tended to be (historically) more of an Irish or Scottish thing - those immigrants taking it over to the US/North America, where it's evolved into more of a big event.
In England, the emphasis is more, I think, on Guy Fawkes Night. Also in northern England, there is what is called Mischief Night - which is actually the night before Halloween.