"Cornwall has lots of transatlantic cables"
I'm glad someone else has noticed that's largely irrelevant.
It's a bit like saying that because there's a 400kV power line at the end of the street, consequently the street can all get cheap 440V three phase electrickery.
As the man on the TV says, "It doesn't work like that.".
Anyway, we know what's connecting these cabs to the end users (shortish lengths of classical telephone wiring) but what's *really* connecting these cabs to their upstream connectivity? In the absence of that info, there's a piece of this picture missing.
It's about as useful as having fibre to the cabinet from your premises, and no fibre from the cabinet to the big wide world. Which is pretty much what the alleged FibreCity/H2O Networks rollout in Bournemouth has got at this stage, ie a chocolate teapot.