Now fair enough about the logistical tail of the BOAR, but I can't see the Spanish border guards stopping hundreds of Chieften tanks with a rifle &/or Spanish swear words. The only garrison duties I thus see the BAOR doing is in Madrid, Barcelona & a dozen other Spanish cities. Then, I'd dare argue, the Spanish govt might consider swapping Gibraltar for the northern half of Spain. That's, of course, if the Spanish population doesn't conduct a democratic revolt, put the post-Franco facist government up against a wall somewhere, & come to terms with the UK.
UK keeps Gibraltar & Spain becomes a liberal democratic nation. Spain joins the Common Market & NATO not long afterwards. The British are seen as liberators until thousands of British tourists flood the country getting completely drunk & making a general nuisance of themselves.