Its a good point. The UK could actually have foreign affairs crisis affecting gibraltar and also the falkland islands. Maybe we need a UK-specific ‘spain seizes gibraltar’ event for if military spending is really low and foreign relations are really low?
One of the hardest thing in Democracy 4 is ensuring that it makes political sense to spend anything at all on the military, because generally speaking if you reduce patriotism enough, you can then gut the military and save a fortune, with no repercussions 
In open debate, nobody would seriously suggest that the only thing stopping Spain from seizing the rock would be UK military power…but it has to have some impact right?
As a UK mainlander, I can attest that nobody I know cares one bit about the rock, and we mostly only care about the falklands because we hate the thought of losing a war