The mandate doesn't apply to free services (they can opt in if they like). However, companies aren't allowed to charge a premium for the newfound freedom. Not that they're likely to complain -- Amazon told Wired that it welcomed the move, as it will "directly benefit" Prime Video subscribers by making their service useful on trips.

As we pointed out earlier, though, the benefit should be short-lived for UK residents. Brexit is expected to deny portability for UK customers as of March 29th, 2019 -- most likely, you have less than a year to catch up on Black Mirror during your continental travels. While you can work around this by downloading offline copies before you leave, that won't help much if new material shows up when you're abroad. It's still nice to get a taste of true media portability, mind you, and the new rules are a marked improvement over the isolated media experience Europeans have known until now.