Is Halloween an American or British celebration?

I think people in both counties celebrate 

It's origin was with the Celts long before Christanity decided it was their celebration and it made it all about them.

It was a Pagan festival about warding off evil spirits and it originated in areas that are now Ireland, all through Britian and northern France. America didn't even exist under white rule when the festival was created. 

Scottish. Then American. Now it's invaded Britain. We celebrated Guy Fawkes night on 5th November, where an effigy of Guy Fawkes would me made and burnt on the top of a bonfire, and here in Yorkshire we had mischief night on the night before, where children would go out and cause mischief. Today Britain's youth have forgotten these things it seems.

Halloween is a pagan celebration observed by individual who don't know what the hell is going on.

Halloween is the shortened name of All Hallows Eve, the eve of the Western Christian feast of All Hallows' Day. It is dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed. In the eighth century, Pope Gregory III designated November 1 as a time to honor all saints. 

The true origin is the ancient Celtic festival of Samhain  (pronounced sow-in), 2,000 years ago, mostly in the area that is now Ireland, the United Kingdom and northern France, celebrated their new year on November 1.

It's originally European, though celebratory details and traditions change over time.