Re: BT Sport Ultra HD
The box Plusnet sell for TV doesn't support UHD so they don't have BT Sport Ultra HD. They haven't announced here (AFAIK) that they are going to start either.
The only partial service you could get is a second hand G5 box on ebay for a HD box to get netflix 4k via a STB, but i have yet to see a decent 4k TV that didnt have netflix app on it natively.
BT Sport and Netflix are the only 4k content I have found on YouView (no youtube or amazon video)