Why wouldnt people self isolating be watching films during the day? What else are average people confined to their houses going to do? This being confined hasnt really happened in the UK yet.
A bit of Netflix or iPlayer will have little impact on the Internet, apart from maybe some of the niche ISPs with limited peering capacity, although even these ISPs already have to cope with evening peaks. And currently, so far this week, daytime traffic levels are significantly lower than the evening peaks on a normal day (pre hysteria, non match days, non new Apple update, non
Red Dead Redemption 2 update etc).
The school closures will have more of an impact, as kids make extensive use of gayTube, and apart for the odd viral vid, no real deterministic pattern, making caching a problem.
But the reality is, even if self isolation would cause the same peak levels as the evening peaks (probably not), the additional working from home traffic, probably would not be enough to exhaust the additional safety barrier capacity that most ISPs have.
The big web conferencing companies have presence on most continents, and are well connected to peering points. Same applies to video streaming services such as Netflix, Amazon, Sky, BT TV, youtube and so on. Same applies to the cloud provides from the likes of Microsoft (Azure, and Office365), Amazon (AWS and S3), Google etc. So that dramatically reduces strain on the intercontinental routes.
The bigger ISPs will have one or more of these services either at their chosen peering points, or even at the point their customers are terminated at.
BT last week (pre the current even heightened hysteria) publicly reported a new record of 17.5Tb/s across its ISP network due to the aforementioned Red Dead Redemption 2 update. Considering the sheer number of (several million) customers - its by far the biggest ISP in the UK - that 17Tb is quite low when you think about it. If you did the maths on that, dividing the bandwidth by the number of connected users (a figure I don't think is public), it averages out to each line being almost idle.