Have you tried connecting your BT box directly to your hub to eliminate the powerline adaptors?
I've never used powerline adaptors myself but I would suggest finding a way to eliminate them just to be certain. I have used the Prime app on my 4k box and it was a bit hit and miss, the app on my TV never misses a beat though. My guess is that the problem lies more with BT/Youview than Amazon but I don't think you'll ever find out.
Workarounds seem to be better options rather than waiting for service improvements.