Jeez, so many seem to ignore the fact that it is the British who are screaming about war, not Spain. But ok, lets play along:
I didn't say it was 3x larger I said it was 3x more powerful - there is a difference. Spain for the most part has relatively basic ships in it's Navy versus the UK having very high tech powerful ships (including multiple larger destroyers) with reduced radar cross-sections etc. That's not including the more numerous submarines which are also superior in technology and power.
You might want to update your knowledge on this topic.
The Royal Navy only has 6x Type 45 air defence destroyers, which are world famous for having serious problems with breakdowns in warm waters (such as those around Gibraltar). They can deploy only 1, at the most 2 at a time. They also have 13 elderly frigates which have very limited air defence capabilities. From 2018 onwards they won't even have Harpoon missiles any more.
In comparison the Spanish Navy has 5 of the extremely potent Alvaro de Bazan class AEGIS destroyers, a ship so capable that even Australia brought it. And unlike the Type 45s, they actually work. They also have 6 older Perry class frigates, but even those are more capable than the British frigates, and are still very capable air defence ships. Spain also has the advantage of operating from home, unlike the British fleet would. Spain also has an operation aircraft carrier complete with Harriers, unlike the UK...
Spain also has the ability to strike the British fleet from air through the use of F/A-18s and P-3s firing Harpoons.
The RN does indeed have some very formidable nuclear submarines, but anybody with a basic knowledge about submarine warfare knows that diesel-electrics are superior boats. Even the crew of the HMS Conqueror were terrified that they were going to run into one of Argentinas WWII-era Gato Balao's back in 1982. A nuclear boat has greater endurance, size and speed, but thats about it. A diesel-electric is significantly more silent, which is what matters the most when it comes to detecting a submarine. In this case, you would even be playing to the diesel-electrics advantage, since it would be on the defence.
Finally, the British fleet would be operating very far from home. It would be overly dependent on it's supply ships.
The RAF does have a larger number of fighter and attack aircraft (and of course is starting to receive the F-35 - which while flawed is still up there with the EF) The RAF also has tankers and AWACS platforms.
Which is insignificant. The RAF would be operating strikes in the length of 5000 km. At those distances, one tanker might be able to drag a single fighter down and back again (and even that seems optimistic). With just 14 tankers in the RAF, they would be able to bring only 14 aircraft to the fight at a time. Now they might be able to increase that to 28 assuming they overflew Spanish airspace on the way to Gibraltar, but that would risk losing the tankers on the way, dooming all the fighters and preventing future sorties.
The F-35 isn't going to be operational for a while either. Britain will be long gone from the EU before that dog is ever operational.
Spain's SAM defences aren't particularly extensive (and with stealth aircraft like the F35 shouldn't pose a problem). Of course the other thing is a few pre-emptive missile strikes on Spanish Air Force bases helps resolve the issue very quickly too.
Spain just isn't that far away from the UK especially with tankers available.
Spain has NASAMS, which is the latest SAM system deployed by any NATO member. They also have Patriots and ASPIDE 2000s, both of which are very capable missiles. All 3 systems are highly mobile, and if deployed competently, can't really be targeted by fighters like you suggest. And as mentioned above, you don't have to kill the fighters, just the tankers on which the fighters are 100% reliant on.
As with everything in this scenario, Spain doesn't have to engage the British forces directly in combat in order to win. All they have to do is to defeat the British logistical chain which is very
brittle and vulnerable. All of this also ignores that fact that Spain could probably overrun and fortify Gibraltar in a day or so, meaning that by the time the British fleet arrives, they have to retake the rock in the face of Spanish defences and support from highly mobile land-based artillery. British landing craft coming ashore would even be subject to attacks by ATGMs launched from ashore. Spain would also have the advantage of heavy armour support, while the British would be limited to infantry. Both side could bring attack helicopters into the fight, but again, the Spanish could deploy loads of MANPADS and other air defences to prevent the British helicopters from taking to the air. Realistically, Spain would also have a complete dominance in the air, and so being able to bomb the British troops from their fighters.
however Spain would definitely come out of a conflict far worse off.
No they wouldn't. Militarily, Britain can't realistically do anything that wouldn't cause a massive political fallout in the aftermath, to the benefit of Spain.
Add to that the 1,000 year old challenge of crossing the English channel, in the face of British Vanguard class submarines, and an army that is not a marching band.
Did you ever bother to take a look at a map? It is the British who will have to cross the Channel this time. And the Bay of Biscay. And unless they want to fight their way all the way through Spain, also the Atlantic ocean.
The US will then nudge Russia to 'pressurize' and create an Eastern Front. Without US support, Germany and Eastern Europe will have barely any military to cover their own butts from the Bear.
Russia wouldn't intervene. A Spanish annexation of Gibraltar would only serve to legitimize the Russian annexations in Ukraine. They be 100% on the Spanish side...