prioritised, thank you

Strictly speaking, the -ise/-ize ending doesn’t depend on whether it’s UK English or US English, but on whether the root of the word concerned is Latin or Greek. The -ise ending can, for etymological reasons, be used with Greek or Latin stems, whereas the -ize ending should really only be used, in strict terms, with words of Latin origin. Unless you are comfortable with classical languages, it is, from an etymological perspective, safest to use the -ise ending. However, if you don’t, and prefer the -ize ending, governments won’t collapse, and mares won’t miscarry in the streets, so use either as you prefer. I use -ise invariably, myself.