JANET went live in 1984. Before then, UK universities were connected by X.25 links with ARPAnet gateways to Rest of World. (Such as it was then - basically the US and Scandinavia.)
You could, with a tiny amount of password hacking, joyride around the system quite easily, at least as far as getting to a login message on a remote host, possibly logging on with a guest account and having a text chat with surprised people in other countries.
Yes, SRCNET/SERCNET/JANET were great as a physics researcher, despite what people have said about X.25. A potentially interesting point is that TCP/IP on JANET originally ran over X.25 until X.25 was finally phased out.
You could, with a tiny amount of password hacking, joyride around the system quite easily, at least as far as getting to a login message on a remote host, possibly logging on with a guest account and having a text chat with surprised people in other countries.