@iota - Perhaps I should've described it as while a country may choose not to aid in the physical enforcement of the other citizen country's laws they can't intervene with the efforts beyond the scope of the resident country's powers (e.g. seizing money that travels through the US). That's the way FATCA is being enforced.
It appears to me that FATCA is being enforced in the UK by being mediated through the IGA.
1. The IRS tells the banks to implement the FACTA reporting requirements, and cleanse their books of any accounts held by a US Person who doesn't co-operate.
2. This would be illegal under UK law, but the banks can't afford to refuse because they're all up to their necks in the US financial system, therefore no protection against the 30% withholding threat. The UK really really doesn't want another banking meltdown, and also really really wants to get in on Automatic Worldwide Big Data Exchange.
3. Enter the IGA: UK-enacted legislation, making it legal for the banks to comply with the requirements of FATCA.
Personally, I don't expect FATCA to be successfully challenged, but I would very much like to be proved wrong.