Hi,
I am in the same boat as you are! I Have a EASA Spanish PPL but I want to carry all my modular training in the UK. As far as I know, I've visited Professional Air Training (and they know pretty pretty well all this stuff), is that you can carry on all your training in the UK and being issued an Spanish EASA CPL ME/IR! The UK CAA examiner will completed a form advising the Spanish CAA that you will take your skill test and all will be fine. Therefore, you have to options now! You can transfer all your Medicals to the UK and being issued a UK CAA CPL or keep your Spanish Licence. I am still thinking what to do but in the Spanish licence you will have the SPANISH LEVEL 6 and your English level (I have 4), so I think it's something that a lot of British pilots don't have, and therefore is worth to have in your licence. However the UK licence (at least for me) has some more kind of "attractiveness". There are some UK airlines that wanted to you to have a UK licence (Easyjet or BA, but they will not hire you, at least as a freshly graduate), and some UK airlines (Flybe) provided that u have a good ATPL marks and done some training in one of their modular partners ATO (like PAT) don't care!
So decision time! Hope this help!