You must be a Registered Resident to import car ISV (tax) free but you must also remain a resident for a further 12 months, otherwise you must repay ISV savings, plus a bit more.

As a Resident you can import and pay ISV, but then you might as well buy a car here, but I don't think the 12 month residence applies.

The catch with all of this is that the car for road tax purposes pays the rate of year of import not the year of first registration in UK, which with the wrong car can have a considerable impact.

If your bringing a car with you then you can keep it in Portugal 183 days in any 12 month period, but new implementation of EU Residency laws only allow you to stay in Portugal a maximum of 3 months, when you must register your Residence then as a Resident you cannot legally drive a UK or any other EU registered vehicle.