I've usually been able to scrounge bike boxes from my LBS (I'm a regular customer).
Twice I've used a local man in a van who has a good reputation, he has a flat rate of 50 e per delivery. This is fine if you are not in a hurry as he waits until he has a full van before leaving. I've used him without problems.
Using my local parcel keep service has been expensive 65e for forks plus they used TNT. The parcel went from Almeria to Madrid, Frankfurt, London Docklands and to Liverpool where the courier attempted to steal it but fessed up after video evidence. More to the point TNT UK blamed TNT Spain and vice versa. Only perseverance by the receiving member on here recovered the situation.
Lately Correos have proved excellent and I see that they now sell bike boxes too. I've used them for shipping wheels and forks recently. For example I shipped some forks to TF Tuned in Wiltshire all tracked and insured for 40e, they arrived in 5 working days.
For shipping from UK to Spain I've found nothing better than Parcel Monkey.
Of course nothing beats moving stuff either way as hold luggage. New rules say you must deflate your tyres though.
Hope this helps.