Driving licences all over the EU all have a very similar design. It's dead easy to swap a British, or Belgian or Dutch driving licence for a Spanish one but lots of Britons (I'm going to stop adding in other European countries now but I'm sure you've grasped the point) prefer the home grown product. So, if by the simple device of getting a friend or relative in the UK to allow you to use their address you can hang on to a driving licence made in Swansea that's what lots of us prefer to do.
How legal a non Spanish driving licence is for an EU citizen resident in Spain has been a perennial bar room topic for years. I had my own understanding of the regs around driving licences and I sent my licence to be changed to a Spanish one last July just before I'd been officially resident for five years. The replacement still hasn't arived but that's a different story.
Laws change all the time in every country and public awareness of those changes is not always high. That's the case even when the changes and the publicity about them are available in your native language. In Spain where most Brits don't get their news from the Spanish media or even Spanish sources two things happen. People either know nothing about the changes or, more usually, several different interpretations of the changes start to circulate.
In our home town in Alicante we are lucky to have a much more relaible source than most. Clive wearing his UCL hat. Clive told us about changes to the driving licence requirements. Resident Brits were offered basically two options. Change our UK licence for a Spanish one or register our UK licence so that the Spanish authorities could find (and fine) us. The cut off date for the changes was 9 December.
Some of our friends here recently registered their UK licences with the traffic people in Alicante without too much bother. Maggie thought she would do the same today but she chose to use the traffic office here in Cartagena. Apparently she caused confusion and consternation amongst the workers behind the counter. They vaguely knew about registering EU licences but they weren't quite sure how so they took all the paperwork that Maggie proffered plus her licence and said they'd give her a call when they had some news back from Madrid.
It seems it's not just the Brits who get their legal updates in a haphazard and individual way.