In Denmark, you can buy a vanity plate (ønskenummerplade) for 8'000 DKK (needs renewal every 8 years), and it can be between 2 and 7 characters long; but the best part is that they permit all Danish letters, including Æ, Ø and Å. One could likely write a script quickly to check these platforms for short combinations, such as ØÅ, which appears to be available.
I'm imagining someone driving in England and the police having no way to input those letters into their system.
I wonder if the Danish system would prevent ÆØÅ and AEOA from both being registered. Would the Danish system Match "ÆØÅ" if someone input "AEOA"? There are unicode normalization rules, but I wonder if systems would be built to handle that. If you're Danish, you'd just use those letters so it wouldn't be a useful feature. If you're English, you wouldn't often encounter those letters so it wouldn't be a useful feature.
ØØ7
Don't forget that the cost is not only the bureaucratic fee; you also have to buy a vintage Aston Martin or Lotus, to display the plate.