The leverage is the ability to make the same amount of money in a non-cleared role.

By leveraging the fact that one is a UK citizen, and applying to SIS or GCHQ, I assume? Most other countries perform in-depth background checks, rather than relying on pseudo-science, when it comes to matters of national security.

The background investigations are still performed, the poly is an additional layer added on, so that's the only real formal difference between countries.

I really want to believe that polygraphs are just used as an interrogation tool and not as a substitute for the SSBI. But the FBI gave Nada Nadim Prouty an SCI clearance even though she wasn't even lawfully present in the US, let alone a citizen. She was working for CIA when the next investigation was due, and they didn't have a problem either.