I have just checked out a site which sells old UK registration numbers – http://www.regtransfers.co.uk It appears that the only letter that does not appear anywhere in a registration is “Q”.

I would have said that for letters which signify the year - O and I are omitted, as are Q U and Z. I imagine Z is usually omitted because no-one wants to buy the last of the old series, they would rather wait until it starts with A again. Suprisingly, there seem to be plenty of registrations issued with all of these letters in the body of the registration sequence. As far as I can see the one with the least number of appearances is “Z”.

The system has recently changed and the letters are changed twice each year, rather than once.