This is not a regional variance issue. Simplifying it as such is obtuse and demonstrates a lack of proper education.

The UK used the -ize form correctly on most words up until World War 2 then started getting lazy and began to use -ise for everything. The Americans did the same, but from the other direction, using the -ize form for words that had always been spelled with an -ise.

This is an excellent article I suggest you read:
http://www.metadyne.co.uk/ize.html