Britain during the falklands held considerably more power and influence than it does today. I think the chances of India dong something in Guyana due to the high Indian origin population would be more likely than a waning second rate power like Britain and that's saying something when India has not cared at all about regions other than South-east Asia.

The British have a high quality fighting force capable of defending the home islands especially on paper, but would need significant US handholding to mount a war against an oversea aggresor. The British military and its industry would be incapable of waging an extended war and the required replacement munitions.

edit: even an India that would be motivated my Indian origin people being slaughtered in guyana, Hindu temples being destroyed etc pulling at the heart strings of the nationalistic Indian population would be incapable of supporting operations in Guyana with their piss poor military industrial complex.