Here in Toronto it's one or more lit Jack o'Lanterns (or sometimes a display with or without sound effects) that signal availability of treats. The little Batmen and Sailor Moons come early with their parents hanging back on the street and if you can get it right you run out of treats before about 8-9.00 when the belligerent older kids come along.

When I was a kid in England we didn't have pumpkins and we used rutabagas - much more difficult! I agree with Stapfam that Britain doesn't really need Halloween - Guy Fawkes Night is just around the corner - bonfires, fireworks, cinder-roasted potatoes, treacle toffee, parkin cake and toffeee apples!