Compliance is a form of taxation...double compliance, therefore double taxation.

Boris Johnson pays all his UK taxes, but the US wants to tax him more for taxes the UK does not have. He pays tax once to England on income but then the US wants to tax that same income in a different way and then it becomes double taxation.
This broad definition of double taxation is much more realistic than the narrow Treasury department definition that works something like this: The US has a new Net Investment Income Tax. Some call it Obamacare tax. If you are in the income leagues of Boris Johnson and even considerably lower you may attract this. Here is how it is not double taxation according to the Treasury Department definition: The US has this 3.8% tax (would not apply to Boris Johnson’s home sale in 2009 as that was before the introduction of the tax), and the UK does not have a Net Investment Income Tax/Obamacare Tax. Therefore the US rate of 3.8% flows on top of all other taxes in full and is not double taxation because the income is not subject to a UK Net Investment Income Tax. This is an example of how the tax treaties the US defined “prevents” double taxation. The reality: the taxes flow straight on top and according to the tax treaty there would be no credit allowed for the US tax paid on Net Investment Income Tax for UK source investments against UK tax liabilities.
Maybe another way to think of it is if there is no credit for US taxes paid against UK taxes for UK source assets/income for those tax resident in the UK then it is double taxation.
The whole tax situation is very complex. There are many including it may appear most politicians who when they hear “tax” they shut down and do not want to comprehend any further. Then when there are two tax systems overlayed on each each other it gets more unfathomable. This is part of the reason politicians around the world have been duped by the treaty language of “preventing double taxation” and think helicopter view – prevents double taxation great, I’ll vote for it! Yet not preventing double taxation for those impacted.
Whether Boris complies or not, his action (or inaction) would be a reasonable response to a law imposed on him by an immoral nation. Whichever route he takes, he’ll be accused of not acting with moral guidance.

It is possible that those in the UK will back Boris – because of disdain for the US, as well as seeing the absurdity of taxing someone in the UK based on having lived in the US till the age of 5.
And as UK trusts get a load of the amount that FATCA certification is eating into their entirely UK based assets – EVEN WHEN NO POSSIBLE US connection exists – then I think they’ll cheer anyone who tells the IRS and Treasury to take a hike; ex. “… New US tax measures, aimed at helping American authorities collect tax from their overseas citizens, are leaving innocent Britons out of pocket….. Although it was established for wholly innocent reasons, this trust along with an estimated 100,000 others now falls within the far-reaching scope of FATCA.
Once the review is undertaken, if the accountant is satisfied the trust does not need to fulfil any further obligations under FATCA, there are no further costs – and no information will be passed on to HMRC or the American authorities. “This whole process seems extraordinary,” said Mr Stewart. “The trust just has a property inside that is not providing any income so I don’t understand why it needs to be reviewed, simply to satisfy regulation introduced by another country.” …….accountancy firms are also carrying out reviews and are charging for their services, with “initial review” fees ranging from £200 to £500….for those people who have set up trusts. They will be contacted irrespective of whether there is any suggestion of a US connection and, as with the Stewart family, they may have to pay their accountant to undertake a review. About 100,000 trusts could be affected including those set up to look after children’s assets or reduce inheritance tax.
The purpose of FATCA is to put the onus on the world’s big finance firms to check whether any of their customers are dodging tax owed in the US. If firms don’t oblige, their US operations will be hit with a 30pc tax. The UK divisions of firms have hastily complied – at customers’ expense. ” http://www.telegraph.co.uk/finance/personalfinance/tax/11050777/British-families-billed-500-to-prevent-Americans-dodging-tax.html
Who says that “only” US citizens and USdeemedtaxablepersons are affected by FATCA? Clearly these legal, local UK trusts with ZERO US connections are paying hefty fees to satisfy the US – and there is absolutely no reason why they should be.