One of the things I don't get is why there's never any mention of conflicts with EU law. If it's going to be a govt to govt (UK-US) agreement, UK law still has to be compliant with EU law. And given the frequency of dual citizenship, some of the americans will be dual US-EU citizens. So, for example, if a UK-US dual citizen is denied a bank account (or whatever) in the UK shouldn't that be the denial of goods and services to an EU citizen.
The EU has already blessed the efforts of the US, UK, France, Italy, and Spain to proceed exploring the government to government information sharing relationship. Government to government did away with any privacy law concerns.
As for denial of service, for financial institutions wishing not to be compliant, it would be more of an inability to meet requirements. A business, in this case the banks for example, could not be forced to take on accounts that would cost the institutions more money or manpower than they are willing to spend. Sorry mate, the job's too big!
The information sharing from the 5 countries depends on two issues. First, the lobbying power of financial institutions in each EU country in persuading their government to agree to the terms, at whatever cost, in order to prevent a possibility of a 30% witholding. Second, the agreement depends on the 'reciprocity' of the US in releasing like data on foreign accounts in the US to the EU partners. As I understand it, this requires an agreement to share information from the US Congress. I don't know where this stands in the US at present, but I do know there was strong opposition from the US financial industry. Has it passed?
If the US ends up not agreeing to release data and there is no reward for HMRC, etc., then the first issue, power of the financial institutions, becomes the main driver.
A big OOPS! I forgot to include Germany. So it's the US and 5 EU partners.