It would be interesting to know if anyone has quantified the economic detriment to the UK of not being in the EU comment market, versus the economic benefit to the UK of becoming part of a US common market. It having a clue as to what the data is, as a guess it would seem to me that it might be close to a wash.
Will the UK have any say in the rules of this 'US common market'?
What rules do you have in mind? Putting aside the free flow of peoples, in my ignorance, I am thinking of a common market without rules. There are no tariffs. No export incentives would apply. But that statement by me likely just reflects my ignorance on these matters.
The fix on the pharma thing, is that drug companies have to charge consumers the same price in both countries. The US should not be subsidizing drug research for the planet. The US should do that ASAP (pass a law that drug companies cannot price discriminate between nations, at least those that are relatively wealthy), and that will mitigate the "problem" vis a vis the UK, as drug prices go up some in the UK and elsewhere, while going down some in the US. Or the EU common market does without certain drugs, because they don't want to pay the freight.