FATCA
you might want to read up on tax treaties, the US FATCA legislation means that the UK is already committed to reporting on any US relevant tax data in a format requested by the US.
From a legal perspective companies within the UK are required to report these transactions to HMRC who then provides them to the US IRS.
My guess is that from a national security point of view tax data just isn't that important (note that the article talked about information classified as OFFICIAL which I believe is the lowest level of government data - see https://www.gov.uk/government/publications/government-security-classifications)
Biting the hand that feeds IT
