There are states in which ethnic identifiers of any kind cannot be used on licence plates. I can’t recall (and a quick Google search isn’t finding) the specific example I’m thinking of, but a couple years ago somebody tried to get something perfectly innocuous on their plate, identifying themselves as Irish or Italian or whatever, and they were knocked back under this law.
I’m not particularly offended and I’ve even called myself a seppo a few times since learning the term a month or so ago but it is pretty offensive in nature – we’re being called shit receptacles.
Selecting one of the plate backgrounds that is offered allows one to take steps to order the plate, including an option to check on availability (I chose “Standard” just to access the page):
LIMEY came up as unavailable, although whether it was already taken or prohibited the response did not say.
(I could not see any Yanks taking offense at the word Limey and if your fellow Brits don’t seem to mind, it would seem pretty inoffensive. Too bad it’s out.
OTOH, Limey was originally a double insult, although the second aspect was inflicted by the Admiralty: once lemons were shown to ward off scurvy, someone in the Purchasing Office decided that limes would work just as well, and limes were cheaper. Unfortunately, limes carry a small fraction of the Vitamin C needed to ward off scurvy, so for around 50 years, British sailors wandered the oceans carrying a mocking name while still suffering scurvy.)
All I can go off of is my personal experience. British expats in Tokyo, HK, China and other places I’ve lived/visited use “septic” a lot. As I posted elsewhere, these included Sandhurst Grads, Oxbridgers, as well as some pretty low life types. One hears it a lot, and it’s mixed with smirks smiles and giggles about “oh how fuckin’ clever we are because the septic here doesn’t understand our slang as we pretend to be his best buddy and make him shout a round.” I could care less about the emtymology, I just know I hear this a lot more often than anyone could possibly attribute to a particular London subculture.