That's the default when you host an app on the world wide web, though. Regardless of how big of a burden it is for 4chan (I would think it's as simple as flipping a switch in some control panel blocking UK access?), it still does compel the US-based company with no commercial presence in the UK to consider complex international law and to make changes to their US-based web app in response to a foreign jurisdiction's regulations, which feels wrong to me.
This is tangential to whether it affects "free speech" outside the UK, though, and I'm inclined to agree that it doesn't, but I guess it depends on how you define free speech. If 4chan's web app itself is considered speech, and not just the content that's posted there, maybe. But I think free speech advocates are a lot more concerned with the content.