2 different shows to phones/tablets = 2x4mbit/sec = 8mbit
Parents streaming video - assume 4K HDR (even though virtually nil content is in that) = 25mbit/sec
Twitch = 2mbit/sec (upload, but for the sake of argument, assume it was download)
Add in 10mbit/sec for browsing, updates etc and you're still at 45mbit/sec.
More than 55% of the UK can get >100mbit/sec currently - definitely not <10%: https://www.thinkbroadband.com/news/7918-january-update-on-s....
I know coworking spaces that operate with around 100 people on 100/100 internet with little problem.
My grandmas old village in probably the most rural area of NW england can get 80/20mbit broadband (very close to cabinet). If you genuinely think that uncapped 80mbit/sec cuts you off, I don't really understand what you're saying.
HS2 is a very important project and I would much rather that be funded than spending billions on slightly faster internet.