I have BritBox as an app on my LG TV and I generally like it, ie the programmes are great.
My biggest criticism is that BritBox crashes with monotonous regularity.
When this happens I have to find the programme again, and to my fury it doesn't resume where it left off - instead it goes to the beginning again. So I have to try then to find the point at which it stopped.
This seems to be a common problem, judging by the reviews online.
However, if you can overlook this problem, and work with Britbox disappearing in the middle of your programme, it is well worth the cost, as the programmes it supplies are great.