Generally speaking you will be fine - the rules state that someone can start an exam up to the first hour of the exam because nobody is allowed to leave an exam before the first hour is over. In a shorter exam, the time you can leave is obviously the end of the exam and from memory, you should be fine as you entered before anyone would have been able to leave (anywhere around the country). Pearson have to be informed and make the final decision but if there was a genuine reason as to why you were late, the exam officer will have reported this. Make sure you are not late for ANY more exams this season though (that means right through to the summer) as if there is any hint of a black mark against your exam numner, it could cause problems.