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.