If it's about language, then why isn't London a huge tech hub (order of magnitude bigger)? They can sell to the US the same way SV exports back to the rest of the world, no?
If a company headquartered in the UK wants to sell to the US they need to deal with a whole new set of regulations, bureacracy, marketing etc.
In the meantime, any SV startup can grow in a huge market and only then bother with crossing the ocean.
And it's not bigger because there are trade barriers between countries, for services. It's far from trivial for a UK company to sell stuff in the US.
I imagine the timezone difference also doesn't help.
Of course that is only half the equation -- better and cheaper labour (than in Berlin or Paris, say) brings you halfway there, but the language barrier for customers in the EU as well as regulatory hurdles still make massively harder to scale to the whole of the EU than to the whole of the US.