Interesting scenario, from this you will get a 2nd Spanish Civil war as you stated, although I could see it being the state trying to keep Spain together whilst Catalan, the Basque region and the Canaries all try to break away and from the British side, if the relationship with the US is damaged enough a strengthening of the Commonwealth, at least with certain countries (Canada, Australia, New Zealand, South Africa and India); all after all have reason to want to create a stable a reliable power block having seen America leave NATO high and dry or be threatened by unstable boarders or belligerent neighbours.