I would suggest something like >LA##LLL
The > converts all letters to upper case
All reg plates (new style and old style - since about 1983) begin with a letter hence the L (A to Z, entry required).
New style plates have another letter next, whereas old style plates have a number. The A specifies Letter or digit (entry required).
New style plates then have 2 numbers, whereas old style plates can have 0, 1 or 2 numbers. # specifies Digit or space (entry not required; spaces are displayed as blanks while in Edit mode, but blanks are removed when data is saved; plus and minus signs allowed)
All plates end in 3 letters -->LLL
Check out Input Masks in Access help for details of the possible characters you can use.
Hope that helps.
P.S. I'm don't know for sure that I've got the possible formats of the plates 100% correct.