> The Geneva Convention dictates who gets diplomatic immunity, and that is based around your job description and rank.

from what I know, this is not correct.

if the US said he's a diplomat or has diplomatic immunity and/or was given that by the US and notified the UK, then he has immunity and it is up to the US and UK to resolve their grievances through diplomatic channels.

for example, many family members of diplomats and government officials have such status in host countries