(I don't think dropping and readding the channel does anything more than checking to see if you have the latest version of the App. BritBox provides a direct check to see if updates are available)
I have an intermittent video dropout problem with BritBox (all programs). It is infrequent so not a great problem. Had the same problem with SLING. Curious thing is that it happens only with my ROKU Ultra 4800 setup. No problems in another viewing room where I have an older ROKU 4600.
My solution for the 4800 was to migrate to BritBox under Amazon Prime. No BB app here which means you lose a better interface to look at BB offerings. On the other hand if you watch a lot of Prime or have other subscriptions under it, you can go quickly to your last watched channel. I cancelled SLING altogether because it was too expensive for what I watch.
Don't bother with ROKU support on this. According to support they are only hosting other people's Apps so they are not responsible for App problems. I guess it is a tradeoff between device cost and providing creditable customer support. That is not legitimate, in my opinion, because if they bill a subscription, they are collecting a fee for that (probably the same answer even if they don't bill) so they are responsible partly for App problems, at least they ought to work with the provider to see if something can be done.