Given the point of Brexit seems to be “sovereignty” which in turn seems to mean “no foreign courts ever not even for trade deals”, I think that even a 10:1 devaluation of GBP vs. EUR wouldn’t make a post-Brexit UK appealing for hosting EU-27 services. And even my worst-case estimates are not yet beyond 1.3:1 devaluation.

Transfers of personal data to non-EU countries are specifically regulated by Chapter 5 of GDPR. After Brexit, Britain is "foreign" as far as the EU is concerned.

It's also worth noting that a lot of Brexiters consider closer political ties and a strong trade deal with the US as an essential element of their ideal Brexit scenario, by weakening Britain's ties to (and dependency on) the EU. In a no deal/hard Brexit scenario, Britain may lack the negotiating leverage to assert any meaningful kind of sovereignty. Many pro-Brexit campaigners have described Britain as being trapped in vassalage to the EU, but a chaotic Brexit could make Britain just as dependent on the US, with everything that implies politically.

Agreed and that's a terrible outcome as far as I'm concerned.

Politically I'm far more in favour of the EU than the US, the idea that we move further towards the US is horrifying.

If that happens I'm out the door.

Same.

A free trade agreement with the US will mean watering down (or 'harmonising' as the spin doctors like to put it) UK regulations to match the US.