Does that mean that, if by the time of our trip, the Pt drivers licences have not materialised then we could be driving illegally through Spain and in the UK?
Yes, you would be driving unlicensed in Spain and France. As far as the UK is concerned it depends on at what stage IMT have processed the exchange.
Once IMT notify DVLA of the exchange your driver record will state your UK licence has been swapped for a PT licence. Until that point your UK licence is still valid so the UK offence would be failing to produce your driving licence rather than being unlicensed. The problem with that is that you will have no idea when your UK licence ceases to be valid so the legal position may change during your trip.
Please can you point us in the right direction for more information on this matter of concern.
Words to the effect it's only valid in Portugal (in Portuguese obviously) should be printed on the temporary driving permit somewhere (they always used to be).
The reason it's only valid in PT is because under PT law it's accepted as an authority to drive (Article 121.7 of the Código da Estrada) but it doesn't comply with the UN Treaties on road traffic to be considered a driving licence in any other country.
O IMT, I. P., as entidades fiscalizadoras e outras entidades com competência para o efeito podem substituir as cartas de condução por guias de substituição provisórias, válidas apenas em território nacional e para as categorias constantes do título que substituem, pelo prazo a definir por deliberação do conselho diretivo do IMT, I. P..