Maybe the US, which is a very large country, but issue an order to the major ISPs (cellular and cable) to stop carrying domestic network traffic and you accomplish pretty much the same thing. Sure there may be some WISPs in the middle of nowhere which aren't affected, but you've accomplished most of your goal.
There's two ways for internet to be shut down.
One is legal -- the government tells registered ISPs "stop forwarding packets". Doesn't matter how good your connectivity is when you simply have to turn it off. Your protection there are courts etc, but it's not technically difficult.
The other approach is extra-legal -- e.g a terrorist attack.
Just look at the effect from a car bomb in Nashville last month, I quote from a newspaper:
> The immediate repercussions were surprisingly widespread. AT&T customers lost service — phones, internet or video — across large parts of Tennessee, Kentucky and Alabama. There were 911 centers in the region that couldn't take calls; others didn't receive crucial data associated with callers such as their locations. The Nashville police department's phones and internet failed. Stores went cash-only.
> At some hospitals, electronic medical records, internet service or phones stopped working. The Nashville airport halted flights for about three hours on Christmas. Rival carrier T-Mobile also had service issues as far away as Atlanta, 250 miles away, because the company uses AT&T equipment for moving customer data from towers to the T-Mobile network.
The area effected was about 100,000 square miles, roughly the same size as Uganda. From one carbomb outside one building.
I think there's only one major Internet Exchange point in Uganda (just like it seems there was one in the south east of the US), but there's certainly multiple fibres in and out. If you're in Mbarara and sending traffic to Rwanda it's unlikely to go via Kampala.
Here's a network map of one provider in Uganda: https://bcs-ea.com/service/uganda/
The UK has a few exchanges, but almost everything runs through Docklands in London - various Telehouse buildings within a mile of each other, Telecity etc. Almost all internet traffic in the UK runs through London, and the sites that aren't are owned by Linx (IXLeeds is the only one that isn't). A major flood breaching the Thames Barrier will cause chaos.