https://www.google.com/amp/s/www.bbc.co.uk/news/amp/business-47377037
The key restriction here is that after making an international delivery to a member state, they are allowed to make only three domestic deliveries within that country before making another international delivery.
With an ECMT permit, in contrast, they are allowed to make only three journeys between countries before returning to the UK and no deliveries within countries are allowed.