10 Meg ADSL2+ here, and 720, 1080p, 1080p (60) work fine, but 4K struggles, as I expected. I don't get any impression it's specifically Plusnet, as the buffering is constant to the demands of 4K.
Mix bag here (80/20) Youtube failed on 4k, Netflix was fine across the entire range - I watch Netflix and it has never buffered - so far - touching a big plank of wood! The results of the rest were all fine too.