The app publishers select which countries/regions the app is available when they publish their apps to the Roku servers (Roku doesnt control this at all).
So you'd need to directly contact BritBox and Discovery+ about access to content in the UK (there are almost always licensing/legal restrictions related to location/country).
As for having multiple streaming devices, thats a good thing - a multiplatform streaming strategy is necessary to deal with issues related to app carriage/compatibility/functionality/availability and bad firmware/software updates etc.
There is no single streaming device/platform that has everything, or the best quality of everything, so if you do "want it all" you'll need to follow the multiplatform streaming strategy as well.
Outside the US, licensing restrictions/limitations are the biggest factor impacting your access to content, so keep that in mind, regardless of platform(s) used.