I would ask at the post office. They ship a lot of bikes, not sure about the UK... but ask there.
It would be an importation of the bike to the UK, so the customs issues (if any) will be on the UK side.. not in Spain. Ask at correos.
If this does not go anywhere, then I would ask at "Mailboxes ETC" in Santiago (search google maps). They ship with private shippers. It will be expensive, but they should be able to help as well.
Here in our office, we do ship a lot of things... but not bikes. Maybe in the future... looking at the price list Correos offers for shipping bikes, we can't even get close on price, so we don't do it...
Good luck!
Ivar