Microsoft + Google + Amazon + Nvidia + Meta + Apple = $630 billion in annual operating income.
They'll react to a change in capital investment faster than anything else.
Wow I didn't know big tech invested so much in the UK!
They aren't making $630 billion per year in money off of those companies, but the operating income means they're getting taxes on that $630 billion (income tax from company and employees, VAT for purchases, etc.) and the personnel working in the UK are probably spending most of that money in the UK (velocity of money theory comes into play here).
The resulting economic benefit for the UK government is enough that they'd notice the drop if all that started to transition away.
Damage to stock value would be the bigger blocker (from both sides of the pond).
Might kickstart some actual competition though, as that happening would create a large hole to fill.