If you could find a Portuguese registered car for sale, in the UK, it would save any problems with re-registering in Portugal.
Just an idea
Two things, a UK Resident isn't allowed to drive a Foreign registered car in UK, unless
* is employed or self-employed in another EU member state
* uses a EU registered company car temporarily in the UK for business purposes
Eu regulations say
"the private vehicle is not disposed of, hired out or lent in the Member State of temporary importation."
Or course if car had been imported into UK and re plated, you could buy, but you would still need to meet the UK ownership and residence requirements (12 months) to matriculate it in Portugal.
It could also be quite dangerous in not being able to check that the car is free of any debt or even the road tax was up to date, which the new owner would find themselves responsible for.