They already track everything going through the undersea cables, why couldn't they start blocking at those points?

Getting the raw data is relatively straightforward, but my understanding is doing real time DPI at country scale without being easily circumvented (and without breaking the rest of the internet you want to work) isn't so easy.

If you want to really make sure nobody in the uk can easily access 4chan (PRC style) it needs some effort, and you're in an arms race with attempts to mask/tunnel traffic so it gets through

Splicing and mirroring a cable is about 10x easier than filtering it.