There is a good case for some sort of in-sourcing when it comes to national systems. For example if, say, a Chinese diplomat toddled on over to you (let's say you are a US citizen and an important one) and asked you to use his country's foremost VPN solution for secure communications. Would you?
Now let's muddy the waters somewhat. Zoom? sounds a bit Anglo/Sax ... ish. It isn't of course. It also seems that Zoom forgot to encrypt their encrypted channels. lol.
There's more to security and stuff than cash. I do the boring old security thing for a living. It is boring but I like boring.
It’s possible the government is a big enough customer that they could get the necessary economy of scale to make it cost effective, but it also means that they’re going to need to develop a bespoke solution that all government services build on which itself is an expensive endeavor (and politically a liability since such a project can fail to deliver and just balloon in cost).
The UK government does not remotely need the same year-round capacity as a top-tier CDN.