Earlier quoted context omitted.
Except we see just the fine. We have no idea how many attempts and warnings to get them to comply were sent first. It wasn't one email, it was multiple emails, multiple times over months. This site makes no mention of warnings and escalations, and ICO at least doesn't normally announce that for individual cases. Though they do put out aggregate stats. When they have fines are clearly shown as arising in a small minor…
There are other examples at least from Germany where no warning or time to rectify was given, just a fine. https://iapp.org/news/a/germanys-first-fine-under-the-gdpr-o...
Which goes to show why the regulators get the discretion to decide appropriate action from warning only to maximum fine. Without context and aggravating and mitigating factors we can't know, which was my point. If a penalty is disproportionate there's well worn appeal tracks.
Other comments seem to point to the small case in OP comment being some guy running a list to harass people, which seems like a huge aggravating factor to me. Maybe he got one warning, maybe in context he didn't deserve even that.