Thanks for the responses guys.
Dublinpilot, I doubt they would allow me to log the time as PIC if it were not permissible, but I'll double check just in case. I'm guessing you mean this is something I should check with JCARC, and not the UK CAA?
With regards to your second point, it's something I will discuss with them in more detail - but the guy who told me I can log PIC is a flight instructor with experience teaching at various places in the UK too so I'm guessing he's familiar with JAA/EASA intricacies.
How about the concept of a safety pilot in general, ignoring my own specific situation? Isn't the whole point of a safety pilot that they take over in case of the PIC failing in his duties in some way? I'm assuming it's similar to a skills test - where the candidate is assumed to be PIC unless they do something requiring the intervention of the examiner, in which case they become P/UT.
The following people in Spain also provide safety pilots for hour builders:
SERVICES | Hour Building Spain
I think my situation would be almost exactly the same - except that the provision of a safety pilot is additionally mandated due to local law as opposed to just as a result of lack of experience with local operational procedures.