Ban encryption without back doors. Some companies would withraw products, others would comply, maybe some others would protest for a while.

After a while the only option left would be steganography

https://en.wikipedia.org/wiki/Steganography

At which point it seems unlikely they'd be able to do anything other than work on techniques to detect steganography (which they no doubt already do)... which leaves us with a steganographic arms race (which has probably already been the case for some time behind the scenes).

I love the idea of using Facebook, Email, Youtube, Instagram etc. to tunnel Tor.

Wonder how difficult it would be?

It's probably already happening just look for slightly different duplicates of the same videos.

The problem here is that tor is largely focused on realtime communications, and stuffing packets through e.g. facebook chat would probably be too slow. It would work great for another anonymity network that was more message-focused with no realtime needs.

That said, you might be interested in tor's meek pluggable transport: https://trac.torproject.org/projects/tor/wiki/doc/meek

Essentially, the idea is that someone buys a VPS hosted on a cloud service with common ingress IPs (such as Google App Engine, Microsoft Azure, etc). Tor clients then connect to it, sending an innocent-looking domain (gmail.com or something) in the TLS SNI header, and then only in the datastream protected by TLS do they expose the tor protocol.

This is of course useless against NSA and GCHQ, which have access to all these cloud providers, but it's very useful for example for people behind restrictive country-level firewalls as the authorities are reluctant to block such large services and it's hard for them to distinguish this from normal traffic.

They couldn't, as this video explains https://www.youtube.com/watch?v=3G8dPAdmyss

If two people really wanted to communicate, they could still do the encryption on paper, and transmit the garbled messages.

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

Mandatory hardware based backdoors, unlimited funding for CPU power to accelerate decryption of intercepted data, etc.

They could make it illegal and require a backdoor on all electronics?

good luck requiring a backdoor on chinese made chips, or trying to ban their import