Hi,
We had a problem with an Extra aerobatic aircraft based in Italy, G registered.
Fl1ingfrog gave an extensive and accurate descriptions of the steps involved. And this includes a fresh annual done by an UK certified maintenance.
Make sure you do this maintenance before you fly away from the UK.
Without this you will not be able to de register the aircraft in UK, and once abroad you'll be stuck.
This is what happened to us, aircraft was already based in italy for 1 year and a half.
The issue of engine being more than 12 years old is not a problem, it is accepted for private use in most european countries, Though I understand that importing any aircraft in the UK with an engine past 12 years whatever the use is not possible.
Good luck in your endeavour, and expect a procedure lasting more than 6 months, up to a year..Spain has a slow bureaucracy maybe not as bad as Italy but still.