Is the warranty of an iPad bought in the UK valid in Portugal?

http://www.apple.com/legal/warranty/products/iphone-english.html

"IMPORTANT RESTRICTION FOR iPHONE AND iPAD SERVICE.

Apple may restrict warranty service for iPhone and iPad to the country where Apple or its Authorized Distributors originally sold the device."

In fact you may find even if you are prepared to pay for a repair Apple will not repair certain devices (likely anything with cellular capability) from one country in another country. Often Apple does not actually repair a device but simply gives you a refurbished unit that is identical to the one you bought. However, a model from one country may not be available in another.

For warranty purposes the countries of the European Union are considered one country. Of course the UK is no longer part of the EU now...

https://www.apple.com/legal/warranty/products/uk-ireland-universal-warranty.html

Apple One (1) Year Limited Warranty

UK and Ireland

"With regard to iPad devices with cellular coverage and iPhone devices, the warranty provides service coverage in the European Economic Area (EEA) countries and Switzerland. "

Do you know how much time I have spent doing research for just this one topic? ;-)

I don't know. I can't find a reference to it. I just know that before there was a Europe-wide policy, but now that the UK isn't part of Europe I don't know if they are still part of the warranty region the way Switzerland is, or if they are out of it completely.