It used to just be for Guy Fawkes' night. (http://en.wikipedia.org/wiki/Guy_Fawkes_Night) except over the years it stretched to include Hallowe'en in one direction and New Year in the other.

Also, I live out at the edge of town on a housing estate full of young people without much money and nowhere nearby to go, so hanging around in parks drinking is a favourite hobby round here. And the shop that sells their beer also sells fireworks.