It's rather that they have 4Mbps for 720p30 to play well. They don't reach 5Mbps hence 720p60 buffers. We could argue about whether that's 10% or 40% of the UK that is affected. I see your point.
It's affecting 40% of the population, everybody with a connection less than 8 Mbps, give or take. 1080p60 is around 6 Mbps, need a 7-8 Mbps connection to work well.
The 20% who had a connection 4-8 Mbps and could do 1080p got downgraded to 720p60. The 20% who had a 2-5 Mbps connection and could do 720p got downgraded to 480p.
Obviously it's not clear cut, there's a continuous spectrum where things will load much slower and start having breaks in playback until you really have to drop the resolution.
Well, we should also consider households having more than one person. Two people watching YouTube requires double the bandwidth, going into 12 Mbps+ territory. Truth be told, this is affecting more than 40% of the population.
