What happens when you get to ZZ999?
Start again at AAA1?
I base this supposition that many preserved Spitfires have early-sounding serial numbers such as AR213 and AB910, but I have no idea how it actually works.
British Military aircraft registrations started in 1912 with '1', blocks allocated to the RFC and RNAS. When they reached 99999, they changed to A1 - A9999 for the RFC and 'N1' for the RNAS. This continued up to J1 - J9999, when they settled on a 5 digit registration number , so started again at K1000. When they reached the equivalent of Z9999, during WW2, it changed to AA100 - we are still in that sequence today.
There were anomolies, such as Naval Aircraft haning 'N' and 'S' numbers for a while in the 20s & 30s, some letters being missed as they looked like numbers, and 'blackout blocks' They were pretty much sequential until a few years back when it all went nutty. (Future Lunxes have been allocated ZZ4 serials)
There are a few books on the subject out there.
http://www.ukserials.com/