Some BritBox shows are released weekly, so not every episode is available at the same time.
If you've reached the latest available episode and see an option to upgrade, it's because the content you're trying to watch is currently only available with BritBox Premier. This may be because the next episode is included in Premier Early Access, or because the title is part of the BBC Select collection.
You don't need to upgrade to finish a weekly series, new episodes will become available with your standard subscription on their scheduled release dates.
As some shows are included in Premier Early Access, which lets Premier subscribers watch selected episodes or entire seasons before they're released on the standard schedule. You can choose to upgrade to watch those episodes early, or simply wait until they're released as part of your standard subscription.
This also applies to BritBox subscribers through Amazon Prime Video Channels.
Here is more:
How can I tell if a show has Early Access?
Shows available through Early Access are marked with a Premier badge (crown icon).
If you're on a standard BritBox subscription, you may see an option to upgrade when newer episodes are still in their Early Access window. Once those episodes reach their regular release dates, they'll be available with your standard subscription.
Do I need BritBox Premier to finish the series?
No.
Your standard BritBox subscription includes the full series. If some episodes aren't available yet, they're simply waiting for their scheduled release dates.
When will the next episode be available?
Release schedules vary by title. You can check the show's page in the BritBox app or website to see when new episodes become available.
If a series is part of Early Access, Premier subscribers may be able to watch episodes earlier, but those same episodes will become available to standard subscribers according to the normal release schedule.