British serial numbers are often given to aircraft destined for export, so that eats up a few (very few, unfortunately). As
VX275 mentioned, Serials are given to the numerous military model aeroplanes that we now have and, as I read it, these can only multiply.
Tengah Type gave us the real clue but it seems to have been lost in the subsequent crayoning. At the dawn of military aviation, the Air Committee Joint Numbering System was created and set the convention. So, unless we gash the Convention in favour of popular fashion, it's probably 3 letters followed by 2 numbers.
UK Serials is interesting.