OK is very common in everyday speech; I hear it from family members all the time. (The expression spread all over the world during WW2 because of American GI's -- it's very easy to say and understand, no matter what language you speak.)
However, I cannot say that I've ever seen anyone from the UK actually write it out. It's used more in casual conversation, and I think it would be just a bit unusual to see it in business correspondence. (It would be understood just fine either way, though.)