I don't think they'd relocate, but that doesn't matter.

They most likely will pull an ARM and hire almost entirely in the US and India instead of in the UK, because competitive British engineers would immigrate to the States, and you can get top tier Indian chip design talent across the spectrum for $30-60k.

It is impossible to immigrate to the US unless your employer is willing to go through the time-consuming and expensive process of sponsoring a visa + green card, which they have little reason to do since they can pay you less over here. I've been designing chips for years in the UK at ARM and elsewhere, haven't encountered anyone moving to the States

Oof. That is rough. I assumed Arm would give the L1/2 option to high caliber employees - it's the norm in India.

>British engineers would immigrate to the States.

I just don't think this is very true, some perhaps but not many.

Secondly, if "the salleries are too low in the UK" is the argument then why would companies be interested in shifting hiring to the states?

The answer is right in your own question. They are shifting hiring to the US because the top talent is leaving the UK, or leaving the field.