

Not all British accents are non-rhotic to begin with. Exposure to the sound and ability to reproduce it, even if not a lot in speech, means that the onomatopoeia, if used, should be the same.
In languages where a sound doesn’t exist, it gets more interesting. In Japanese, bzzz is not pronounceable and for a buzzer (or something like a phone in vibrate mode) they will say ブー (buu) which is just the syllable bu with a long u sound (think of a crowd booing, but the o vowel there is different to the Japanese u vowel).
I would argue it depends upon the buzzing device, but bzzz for all of them is indeed arbitrary. Even IPA doesn’t represent sounds that humans can’t produce, so it wouldn’t suffice, but them’s the breaks.