a Spanish IR isn't worth the paper it's printed on compared to a UK IR
The Spanish IR is prob99 less "formal" than the perfectly gold plated UK one but (a) we are talking about a probably experienced pilot doing a conversion for a reason no other than the carriage of duplicate papers being mandated (or proposed to be so) by EASA and (b) I cannot see the operational significance of flying NDB holds with stuff like the gates etc, which literally nobody does today in today's
RNAV enroute/terminal IFR environment.
It is time the whole post-WW2 edifice moved on to the latter part of the 20th century.
The advantage of doing this kind of paperwork collection exercise in Spain is that you go there, get your head down and get on with it, and the weather is not going to mess it up. Whereas if you dedicated say 2 weeks to it in the UK, you probably won't achieve it, due to wx.