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.