However, if we got a free trade agreement with the US would the inverse be true, EU companies are better off moving to the UK due to brexit? What about just a FTA with all commonwealth countries (why haven't we done this??)?
A FTA with the USA would come at a significant price — the UK would be pressured to accept low-quality American agricultural produce, and lower many other standards from their current European level. If it does this, that reduces the global value of British exports.
iirc Trump did say he wanted EU to accept our livestock to reduce the trade deficit, leading Lutnick to memorably proclaim "They hate our beef because our beef is beautiful and theirs is weak!"
It's not something that will be forgotten easily. British people on all sides were against reducing food standards.
And then today all the tarrifs are suspended (down to 10%) so it's hardly like there's a reliable advantage there.
We are literally in the process of moving some of our operations to LATM to avoid this uncertainty.
You have to assume that the US is sane a maximum of four years in a row and in four years another Trump 2.0 will again be completely ignoring all their agreements.
In my opinion, the US cannot be trusted with any treaties ever again the way they currently are.