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If you are seeing ads for the BritBox channel on your Roku device, it's likely that these ads are being displayed as a result of targeted advertising. This means that the ads are being shown to you based on your viewing history, preferences, and other demographic information.
Roku collects data on the channels and shows that you watch, as well as other information such as your age and location, to help target ads that are more relevant to you. If you have watched a lot of British TV shows or movies in the past, it's possible that the ads for BritBox are being targeted to you based on that.
It's also worth noting that some Roku devices and channels do come with advertisements, even if you are not using a free version of a channel. Additionally, many free channels on Roku rely on advertising revenue to support their content.
If you do not want to see ads for BritBox or other channels, you can try adjusting your advertising preferences in your Roku account settings. However, keep in mind that this may limit the availability of some content on your Roku device.
We hope it helps!