
@Mag23
As a private academic writer with almost 3 decades experience, I can tell you that being able to write in both American and British English is practically a job requirement.
I don't know about the other writers here who have worked with UK based clients but, I always make sure that I click on the right English type box in my document encoder.
I have to make sure that the proper English spelling is used for specific types of papers. The spell checker helps me to make sure that I am spelling the word in the correct form (British or English) and that the paper is formatted in the correct manner as well. I will be the first to admit that aside from learning how to spell in British, I also went the extra mile to familiarize myself with British and European culture, history, and current events.
I did that all in an effort to ensure that I would do justice to the papers that came my way from British or other European clients. So when a client used to come to me from the UK, either privately or through my academic writing company partner, I was always able to provide the required type of writing and formatting that the student needed for the paper.