If the simplest solution is usually the correct one then yes, the answer is probably that the British simply failed to intercept him. But every now and then an alternative, more complex chain of events can turn out to be the truth. It is perhaps in the nature of Sparrow's work that she does often tend towards a 'conspiratorial' explanation for events. In some cases I found her evidence and reasoning to be fairly convincing, in others I was more skeptical. In this particular case I will need to have another look, which I unfortunately won't be able to do until next week.

Regards,
Chris