If an Islamic terrorist is apprehended in Detroit or blows himself up in Stockholm, it doesn't usually take long to trace their career progression back to Britain. The CIA despairingly refers to "Londonistan", but the phrase doesn't quite do justice to Britain's ability to incubate terrorism all over the country. For various reasons - chiefly our being quicker to accept asylum-seekers than expel villains - Britain has ended up as a kind of finishing school for jihadis.