Common sense guidelines about using US versus UK English
it sounds glaringly obvious but if you have a dotcodotuk site use UK English. And if your site is mainly US-focused, use American English
if it isn’t immediately obvious where your visitors and customers come from, find out from your webstats and write for the majority
What if you’re a global business?
If your customers live all over the world, you can’t really win. Luckily the biggest difference between UK and US English is the spelling. While you’ll probably annoy a few people no matter which you use, the differences in spelling won’t mislead.
Sometimes the same words mean completely different things. Americans might feel comfy wandering the streets in pants and vest, but in the UK we’d probably be arrested for it! Public school, brackets and first floor have different meanings too.
If you can, stick to words that mean the same thing in both countries and sidestep the contentious ones. There’s usually a perfectly good work-around if you think creatively about copywriting.