I was allowed to chose from a few letters 3 years ago. They had the pre fix AA18***, they last 3 letters you could choose from a range of about 10-20. So the location AA18 would be fix and then the next 2 letters e.g. BC would be fixed then you could choose the last letter. So AA18BC would be fixed and then the last letter I was given was to choose from A-T for example. So I could choose S and then the number plate would be AA18BCS.
Hope than helps. But you could ask your dealer.