BritBox is ad-free.
From time to time, you may see a trailer for another BritBox show or film before your program starts. Don't worry—it's not a commercial ad. It's a pre-roll (streaming loves giving simple things fancy names).
If you see commercial ads while watching, you're likely streaming through a partner platform, such as Prime Video Channels, and are not signed in to your BritBox subscription through that partner.
In some cases, the program you're watching may not be from BritBox. Some streaming platforms also offer content from other providers, which may include ads.