I'll just add here that I have a degree in Linguistics and Phonetics and actually wrote a 5,000 word essay on the use of 's' and 'z' in British English (I'll email it to anyone who wants it!!!)
Bottom line is: can use either, technically 'z' for US and 's' for UK. However, technicality has nothing to do with type ins and due to more Americanisms in the UK, it is likely most people in England use a z anyway!