Is warranty valid in Spain on IPhone bought in UK

Generally no.

iPhone warranties contain verbiage to the effect that they may limit warranty coverage to the country of origin.

And since UK is no longer EU …

That said, participants here have posted of successful out-of-country support; so local managers apparently are allowed some discretion in the matter.

“… Apple may restrict warranty service for hardware products to the country or region where Apple or its Authorized Resellers originally sold the device …”

Source: Legal - UK Ireland Universal Warranty - Apple

This is the current UK iPhone warranty document, and it definitely includes the "may restrict" language to which the other responders have referred:

Legal - UK Ireland Universal Warranty - Apple

As much as any of us would like to get you a helpful and binding answer, we cannot. We are not Apple but other end users, so cannot give you an "official" answer. Neither I would trust a store employee other than a manager to give you the correct information. I would contact the Apple Corporate presence in Ireland. That is as close to an official answer as you can get;

It's at the discretion of the people at the service center. They likely won't have international iPhone models on hand for replacements, and therefore aren't obligated to honor warranties or AppleCare plans from outside their own country(except when both are in the European Economic Area or Switzerland).

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