Thanks for posting in the Roku Community! I'd be happy to take a closer look to see how we can help.
Yes, BritBox has not yet been made available by the channel provider in the UK on the Roku platform.
I understand that you are trying to access Britbox using Airplay with your IPad, is that correct?
Please make sure that your Roku 3(4200) is running the latest Roku 10.0 OS by going to go to the home screen and select Settings > System > About.
If you're still experiencing an issue, try checking for any system updates in Settings >System > System update > Check now.
You may refer to this link: How do I update the software on my Roku® streaming device?
If you are still having any issues after following these steps, please provide the serial number of your Roku device so we can assist further.