In particular, I believe a bunch of UK postpaid SIM operators have packages that let you use your UK SIM in many countries at little or no extra charge. 3 comes to mind.
If you'd prefer to maintain a Portuguese SIM as your primary, check out the offers from Portuguese companies. I bet Vodafone has something decent. Have you checked the websites of the three Portuguese companies you mention?
Generally I suspect you'll have better luck with postpaid plans, since the companies tend to be more flexible with what they let you do since htey know they can bill you.