Ah yes, the Pond, also known as the Atlantic Ocean. On one side is the UK and on the other, the US. Each one has its own variety of English, and so clients will require that you use one of the other in your work depending on their location and needs.
Does this mean that you need to be either American to write US English or British to write UK English? I never have and have never been called out on it. Why is this?
For the simple reason that what clients want isn't all the nuances of each version of English, they want correct spelling and one or two words that differ between the two. What is important is to create a list of spelling and terms before you start the translation/proofreading job. Then, during the translation and after, go through your list and make sure you have followed it to a T. Here is a fragment of a possible list:
honor, behavior honour, behaviour
authorize, optimize authorise, optimise (there are many -ize/ise pairs)
September 29, 1983 29 September 1983, 29th September 1983
Instead of saying "Macy's is located in New York (US English), UK English speakers would say: "Macy's are located in New York.