That's why Ofcom started the correspondence, to inform 4chan of laws it may not have been aware of.
> Someone in the [US] can't expect to have reasonable influence over the laws in the [UK] that they're now required to follow
UK companies comply with US laws all the time if they want to continue serving US audiences. I wish this wasn't the case, but this isn't new. Similarly, lots of US news websites aren't available in the EU/UK because they don't comply with GDPR. None of this is new, there's lots of precedent for it.
> You're still trying to apply rules for jurisdiction, that don't map well to the Internet
Sure they do. When I go to boards.4chan.org, the server recognises my request, including where it's coming from, and returns some content. Similarly, when I buy lemonade from a company, they see my shipping address and ships the lemonade. Seems to me like it maps pretty well.
> To call internet geolocation complex or error prone would be an understatement
All other service providers have imposed IP-based limits and that has satisfied Ofcom, so no need to make it more complex than necessary.
> Legal or not, unreasonable demands deserve rejection.
Of course, 4chan is free to reject the demands, just like The Pirate Bay (based in Sweden) have rejected demands from the US government, that was always an option. Ofcom is making the demands to then be able to enforce the OSA, for example by blocking 4chan, without 4chan saying they were not aware of the demands.