“Word was going around that they pretty much had a standing deal with the Brits. MI-6 would spy on our citizens, and we would spy on British subjects. Then they’d just exchange notes.”
That was in Puzzle Palace. Since we could not spy on ours, and they coud not spy on theirs, this was a way to get the job done “legally”.
Brits are “subjects”;we are still citizens although too many don’t care.