How would they realistically prevent people from using encryption?
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).
Wonder how difficult it would be?
It's probably already happening just look for slightly different duplicates of the same videos.
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.
If two people really wanted to communicate, they could still do the encryption on paper, and transmit the garbled messages.
You can make it illegal but it won't stop people from using it
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.