While WTO negotiations in the Doha round largely failed, China perfected its firewall.
When Turkey's negotiations with EU failed, Turkey tightened its firewall.
When it becomes increasingly likely that Britain will crash out of the common market, British censorship emerges.
Whether a firewall is justified by concerns with pornography or terrorism or something else, those concerns tend to be kept at bay as long as economic forces wanting to keep society open are stronger. When those economic forces collapse, political forces that want censorship find it easier to prevail.
Also, import regulation consist of many measures and countries don't just impose those measures on products that compete with domestic products. Countries impose tariffs on wine even if they don't produce wine or competing liquors themselves.
Once the UK has a firewall, they could also decide to block Google out of protectionism. Or (perhaps more likely) they could block smaller foreign companies with less political clout and more UK competition.
Of course, I hope they don't; it'd seem like a pretty bad decision to me. But the current conservative government doesn't have many tech-literate members - witness this very proposal! - so a bad decision is entirely possible.