"S" all the way for me, rather than "Z". Dictionaries may say "Z", but I remember that when I was a kid looking up rude words, our dictionary said that "shit" meant "expelling gas from the anus" or some similar phrasing, which kind of exposed how accurate dictionaries sometimes aren't.

McGrath, the problem with "one" is that unless you happen to be a silver-spoon-in-the-gob upper-class individual, it's difficult to get away with saying "if one climbs a mountain". An instruction manual that says "one must insert screw A into hole B" is shouting at the top of its voice, "English is not my first language!".

To be honest, if you do a spelling check in your preferred word processor with the language set to UK English, to filter out "tire" and insert "U" in the appropriate places, it's highly unlikely that there'll be anything wrong. The variations in "English as she is spoke" around the UK are so wide that no-one's ever going to notice. The only difference after that is in things that have different meanings in the two countries - pavement/sidewalk, boot/trunk, etc.. Grammar is really not something you need to worry about.

Graham.