Regular readers will have notice that I usually use American spelling rather than British spelling on this blog. The reason is that the majority of my 300-500 daily readers are American. I think that the different spellings are pretty well understood whichever side of the Atlantic one comes from, but I have been asked about 'ise' and 'ize' endings. In America everybody uses 'ize' but either is acceptable in Britain. The Times, the Oxford University Press and the Cambridge University Press all prefer 'ize' but all the other newspapers and most other publishers use 'ise', perhaps taking their lead from Shakespeare. In King Lear, Kent exclaims, "Thou whoresome zed! Thou unnecessary letter!"
The ultimate source of the ending is the Greek '-izein'. If the word comes to us from Latin it is from '-izare'. For the 'ise' ending we must blame the French who adopted 'iser' for verbs from these origins.
There are, however, a whole chunk of words whose origins can never be traced to the Greek '-izein' which must always be spelled 'ise'. Here is as complete a list as I can muster: advertise, advise, apprise, chastise, circumcise, comprise, compromise, demise, despise, devise, disfranchise, enfranchise, enterprise, excise, exercise, franchise, improvise, incise, premise, revise, supervise, surmise, surprise, televise.
It is because the list is hard to remember that some people adopt the universal 'ise'. It is not that hard to remember. The 'vise' words generally come from 'videre' to see; the 'cise' words come from 'cidere' to cut, the 'mise' words from 'mittere' to send and the 'prise' words from 'prehendere' to take hold. 'Despise' comes from 'specere' to look (down on). The 'franchise' words come from the French (naturally) and therefore are 'ise'. Advertise and chastise are the remaining two and those you have to remember.