Who has confirmed that they will accept the Spanish training? You need a letter from the UK CAA confirming this, otherwise you may end up having to do the whole lot again. - If however you want a Spanish issued JAA CPL, you need a letter from the Spanish CAA accepting your UK JAA ATPL theory in lieu of the Spanish equivalent.
There are no conversions between JAA countries now, but you can change the state of licence issue.
A totally different matter for any rating (IR, ME, TR, etc) which can be done with any JAA CAA and added to any JAA licence.
cheers IP