Why isn't King Edward The Martyr known as Edward 1st? He was King in the 900s and Edward 1st was King in the 1200s. Edward the martyr's half-brother was Aelthelred 11 so numbering had been introduced at that time.
Reading that back, that doesn't seem to make as much sense as I though it did....
The current monarch would be simply known as King or Queen 'Whatever' with all previous incumbents given a number to distinguish them from one another.
Jack, that's what usually happens in practice - when she pops up in the news it's simply as "Queen Elizabeth"; you only add "the second" in the most formal settings; but her predecessor is (now) always Queen Elizabeth the First.