When the Portuguese investigation was archived there were unanswered questions around the timeline. I can think of no reason why the Portuguese wouldn't co-operate if OG wanted to find the answers to those questions. They would be more likely to welcome it and co-operate fully. They would be highly unlikely to hinder them.
That leaves senior people in the UK who were likely to tell the DCI running the show "what he could look at and what he could not". They made it quite clear in the remit.
From memory - so could be wrong but wasn't it a potential REVIEW that was in the offing when Colin Sutton had his convo with another officer- and it would only be after that had been concluded that it would be decided whether to reinvestigate the case or not.
So surely no remit existed at that point.
I'm not suggesting at all that the Portuguese authorities would not co-operate. However the bureaucracy legally required to be followed by the PT in that co-operation - would inevitably slow everything down. If the other officer knew Colin Sutton's methods of policework well - then it could be he simply thought this case would not be his cup of tea.
AIMHO