Orlena:
Yes, but it was many years ago, I had a U.K. qualification “convalidated” in Spain. At that time there was no requirement for the “Certificate of Experence”.
I do remember it was a very lengthy process in any case.
You might do well to consult with the Spanish Consulate in the U.K. and ascertain what kind of certificate of experience might be acceptable. I am thinking that if you get together signed documents/references from your employers to date, (and that means documents on their letterhead, and with a company stamp if they have one). it is possible that if these documents are then notarized by a Notary Public in the U.K. that they might be acceptable as the Certificate of Experience.
But do find out first before going to any expense.
Speaking of notaries I have dealt many times in the past with Cheeswrights Notaries Public (London) and they are excellent as they have a Spanish department. You can google them for contact etc.
I might add that back then I used the services of a “gestor?a” in Madrid, (Gestor?a PONS), and even so it took a long time.
All the best
Patricia