From the many non-Spanish drive who here in Spain, and the lack of knowledge of those traffic laws which differ from those in UK; there is a good argument that all Brits should take a test before they are allowed to drive (even a hire car) in Spain.
But on a more personal note. My wife has a Non Professional RP licence although she did not even know what the hand brake was when she drove 20 yards in our underground garage. But the problem here is that a learner can ONLY drive with a driving school and the first step is to pass the theory. Thus one cannot get any idea whether a person may have any ability to drive until they have spent a lot of time and money on the theory and even then they cannot practice other than with a driving school.
I have given my wife a few lessons on a road which is not open to the public, but if I cannot take her out on a regular basis (as I did with my ex-wife in UK, she was hopeless but without any driving school time passed her test first time). If I could, then I think if I could get her to a level where she might eventually pass the test.
If we change her RP licence for a Spanish one then I could do that. That is why I am so interested in the possibly of her being able to exchange the DL