"Just enough to have a few guards standing outside Buckingham Palace would do me"
In which case, thank goodness you are not in charge of our country's defence strategy.
The whole point behind a military capability is to deter potential aggrssors, and to be able to defend ourselves if the deterrent factor doesn't do the job. It has nothing to do with empire.
"We are no longer the leading World Nation we once were"
Nevertheless, we are still ranked 6th in the world in terms of military capability.