The UK IMC Rating is a UK-only National Rating, and the UK CAA can only attach the Rating to a UK issued EASA Licence, in the form of an IR(R) (Restricted Instrument Rating)
I believe that the UK FRTOL is a stand alone qualification and doesn't have to be attached to a Pilot Licence, so you could apply for one of these individually, although I'm not sure what benefit you would gain over what you already have.
If you really want an IMC Rating/IR(R), bearing in mind that the privileges can only be exercised in UK airspace, you could always have your Licensing State of Issue changed from Spain to UK!
MJ
Ps. I wonder what has happened to all the non-UK JAA Licence holders who had added IMC Ratings to their Licences? Did they loose them when they had to change their Licences to EASA?