Although Britain & Portugal use basically the same TV system, PAL, the sound spacing is different. Continental Europe uses a spacing of 5.5 MHz, and the UK uses 6MHz. That is why you have the effect you describe.
You may find a TV repair shop can convert it for you, but it will depend on the make, model, parts availability and the depth of your pocket, to say nothing of the willingness of the shop.
I would advise you to look carefully at the economics of this conversion.
Also, check the tuning range on your UK set. If it is marked 21 - 68 you will not be able to receive any Portugese VHF (band 1 & 3) broadcasts. If it has a tuning range of 1 - 68 (or 69), you will be able to.
Lee
March 2007