Regarding the headline here:
US and UK at odds over security tactics as row escalates
something comes to mind.
The signals intelligence alliance between the UK, USA, Canada, Australia, and New Zealand is also an information security alliance. This may be the most important element of it. The countries involved, plus some other partners, maintain a big book of standards known as IRSIG (International Regulations on Signals Intelligence) which sets the standard operating procedures down.
First of all, this explains why the British (or, say, Canada) would care so much. There is no difference between the political decision to share intelligence, and the administrative one to classify it at a level that permits the recipient to see it. To do the latter implies the former, and vice versa. Therefore, most of the information in the system is as classified in the UK as in the US (or Canada).
Secondly, I wonder if there is a plan set out in IRSIG or a similar joint document on what to do in the event of leakers. This would explain a lot.