Well in my example above, it would be assumed that the Spanish will know that the UK will want to get their men and ships out of the now sealed of Med, so will garrison and reinforce the area.
The UK with a weak land army has troops fighting in Africa, India and needing to maintain a strong homeland garrison, as they are under threat from a German Operation Sea lion, can not muster enough troops for a mainland invasion of Spain.
Especially as they will need to pull most of the homeland fleet out of Scarpa flow, in order to help with air supremacy and naval gunfire bombardments on the damaged Gibraltar and the other coastal forts along the edge of the Gibraltar Strait in southern Spain, in order to assist the trapped Mediterranean fleet to force passage through the strait.
Unless the Spanish player has spent resources on coastal forts in the south of Spain, I don't see that they would be in a position to block the strait completely with only a damaged Gibraltar.
This would allow the UK player to attempt to break out under the protection of local air superiority from the fleet carriers and overwhelming firepower from the numerous British warships.
The Spanish player however should still get the opportunity to damage this fleet if they have spent resources constructing coastal forts along the strait, modified by counter battery fire from the British warships.
I understand this is not going to happen for the initial release of HOI4, but it is food for thought for future releases, if the game is going to continue to advance in tactical options and alternate possibilities.
Or maybe it will just go down the more abstracted/arcade route? I guess we will have to wait and see.