Big loss, that destination.
It would be a hassle though.
If an entire continent was at stake, this would be a different story. But, in the end, the UK is small in the grand scheme of things. Any website operated outside the UK won't care, and actively demonstrating this is pretty illogical from their part.
Imagine being a US citizen, and suddenly being banned from Texas and Utah. It's not like you were planning on visiting those, right? Just remember to never accidentally take interstate 10, 15, 20, 40, 70 or 80 when driving around, that can't be too hard.
Suggesting it is okay to have arbitrary freedoms taken away because "you wouldn't use it anyway" is a very slippery slope. Who needs privacy and free spech, not like you want to badmouth the eternal supreme leader anyway.
It also very much seems like Nishimura gives a shit by virtue of the active effort he is putting into his legal defense, consuming both time and money.
Edit: and nobody realistically could be
This is not a fictive fine, it's threats of imprisonment, and ignoring the whole thing means having to avoid travelling to or through the UK for life, and that's assuming the UK doesn't try to activate any sort of extradition agreements.
Even without going to prison, that's a permanent and quite significant theft of freedom of movement. If you ever travel abroad, you could end up accidentally booking a transfer through the UK.
No one ends up unintentionally transferring through Russia anytime soon. And likening the legal threats of a foreign nation to a joke from your neighbor makes no sense.
The UK is a misguided democracy within the usual group of countries considered "the west", enforcing stupid and broken laws in a highly questionable fashion that presents fundamental questions about jurisdiction in the modern world.
However, it is acting against an entity is cancerous enough that even the defendent is purely challenging (and getting support on) the technical legal grounds in a search for precedent.
Even more unlikely is the crown exercising the kind of power you're talking about. Never mind that Charles isn't the King of the majority of Commonwealth countries.
[1]: https://www.reuters.com/world/europe/belarus-forces-vilnius-...