I don't know the 'official' rules and I am not interested in an argument or daft PMs telling me how clever the PM'er is and how dumb I am , this is like everything else just an observation of my own real life experiences.
We went to a British Consul meeting a couple of months back there was a chap from Brexpats brexpats.es) who posed an interesting question ' Who could pass a driving test in the Uk in English?' No hands went up, I couldn't.
He then went on to suggest that passing a Spanish driving test in Spanish might be a smidgeon trickier and that we would need a Spanish licence in 'n' months. 'n' was a number that may have been bigger than 12 or not. I am sure an expert will let me know. My fear was that if the dreadful, awful, stupid, ridiculous 'B' word actually happens then our UK licences might be useless.
So, we have had the medicals (the 'rules' say we didn't need them but the licence feds said we did), our lawyer sent our licences, photos, and 800 documents off to wherever it was (so much paperwork, I have lost track) - we have temporary permits and are waiting for our Spanish licences.
I don't know if we actually needed to swap our licences, who knows? Maybe its worth thinking about if you don't have a Spanish licence?