You can have a Spanish JAA Instructor carry out the 1 hour flight and sign your logbook. You can have a Spanish JAA Examiner sign the licence. For simplicity send a copy of his Examiner authority with an SRG1119 to the UK CAA.
I do this the opposite way around renewing the type and class ratings for our Spanish pilots and established from both the UK CAA and the Spanish that it was acceptable both ways.