The Collaborative International Dictionary of English v.0.48:

factorise \factorise\ v. t.
   1. to resolve into factors, as of a polynomial; same as
      factorize. [mostly Brit.]

   Syn: factorize.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

factorise
    v 1: resolve (a polynomial) into factors [syn: factorize,
         factorise]