With the FAA algorithmic-derivation scheme for ICAO hex addresses you can have some 'fun' by picking an N-reg that encodes to a specific hex code. N22AE gives A1DEAD, for example. Or perhaps N2189N / A1DADA for Daddy's birthday present.
The CAA doesn't use a mapping algorithm. Actually, with every UK-registered airframe now being issued an immutable hex address I wonder for how much longer the CAA will persist with registrations, which are essentially duplicate unique-IDs now.
Here comes G-4070D1 !