Spoiler: The video shows two people using a one time pad to communicate. It doesn't show any reason the government can't ban people from doing that, or stop WhatsApp using good encryption in the UK, or throw people in prison for using PGP. Admittedly that's not going to happen in the UK in the near future, but contrary to popular belief there's no reason you can't make maths illegal.

But it's really quite hard to make using a one-time pad illegal, and even harder to make maths illegal, so practically, while they could install all the backdoors in the world, they would struggle, practically, to prevent people ever encrypting anything.

I doubt they're that bothered about people using one-time pads on paper. I think they're bothered about encryption becoming sufficiently normal that the stuff they want to see is indistinguishable from cat photos. There are measures they can take (legislative and otherwise) to prevent that scenario happening.

Paper aside, I don't see how it would be viable for UK to try to enforce back doors all electronics, ban the import of non-back doored electronics, or enforce a ban on encryption software.

You can make it illegal but it won't stop people from using it

There's also no way they can stop all drug deals, bank robberies and murders, but they still legislate against those things, and I believe that legislation affects people's behaviour.

People will probably always be able to get together in secret underground clubs where they play with one time pads for kicks. This is not what the government / MI5 are seeking to prevent.

If they are trying to prevent major crime or acts of terror with this, it seems like it's doomed to fail - those people will not care whether encryption is legal or not