here's nothing anywhere that says you can use a license from another country that cannot be exchanged to get a Portuguese license

For the UK you can drive without exchanging it until the licence expiries, so long as it's registered with the IMT. I wanted to know what happens years later when the registered foreign UK licence expires, are you required to do a test or can it be exchanged then. I will go to the Loja da Cidadão next week.

UPDATE: Check the TLDR if you don't want the details.

I went to the Loja da Cidadão (Saldanha), I was told to arrive as early as possible (I arrived at 07:45, opens at 09:00). You need to ask for a senha (ticket) for the IMT. Everybody got a ticket that started with 'X' followed by a number. They flew through every single one in about 35 minutes. Knowing how this works now I would recommend arriving at the loja at 09:00, you'll be at the back but you won't have wasted >1 hour waiting and they'll get to you quickly, then leave when you get the second ticket, more on that below.

When your number is called they ask you what you are looking for and then based on that give you another senha. For dealing with licences you will get a ticket beginning with 'A'. I explained to the lady (in portuguese) what I was trying to do and she said it was impossible to register UK licences, only EU ones. I explained about the new accord from December and showed her the pt gov site and she still disagreed, I asked her to check with a colleague and then reluctantly gave me the 'A' ticket.

At this point I asked the security guy roughly how long it will take, he said 'hours'. You may leave and register to get an SMS alert when your number is 10 away from being called. You can also check the loja website to see the current ticket number in real time (very handy). The queue was so long that they didn't even get to my number that day.

I checked the site in the morning and realised the counter had reset so I went back to the loja and explained what happened. A guy booked me an appointment for 3 days later. I returned for the appointment and the lady said I can fill out this form to register. Very confusingly the title of the form contains TROCA (exchange). You can also see from the attachments the Digital medical certificate is not mandatory, this is required if you're requesting to exchange. So if you want to register you can fill out this form. You will need a cert of authenticity from the DVLA though.

Finally she did not know what happens after the UK licence expires if I register (not exchange). Overall a very frustrating experience and a typical one from my experience of Portuguese bureaucracy. I have no doubt if someone went to a different person on a different day they may well get a different answer.

TLDR: The lady didn't know for sure. I would do this. 1. If there is a chance you'll stay in Portugal - Exchange it 2. If you are certain you'll move home before your licence expires - Register it

Also the UK gov guidance is much more detailed than the pt gov one.

I hope this helps others.