So I have had a look at Fly In Spain in Jerez, and they said I have to do the whole MEP course again (4 hours + 2 FNPT + written exam and Skills test) to revalidate my MEP. Would this be true for the UK too?
It's not even true for the rest of Spain. The amount of refresher flying needed to reach the level of proficiency required to pass the MEP proficiency check is
entirely at the discretion of the Head of Training of the ATO. There are no minimum requirements set by the Regulation and the sort of hogwash that you are being fed is designed to only swell the income of the ATO concerned. Any reasonable training provider will assess the training required by reference to your logbook and, perhaps, a session in an FSTD and only then determine the training required. However, the actual time taken may be more or less than the initial estimate according to your ability to meet the required standard.