And Venezuela isn’t, but is an ally of Russia - which would love the USA to have yet another war on another continent to further distract the USA, and Europe, away from the war in Ukraine.
Not sure what the UK could do even if asked for assistance - we used to have a handful of Harriers and a few troops in Belize to deter Guatemala, but I’m not sure we could even raise a few F-35Bs and troops for Guyana, or how much of a deterrent they’d be.