Hopefully to clear some things up about polygraphs in America:

1. You almost never go through them unless you're in the process of obtaining a security clearance (I'd be quite surprised if the UK didn't do something similar for their clearance process). In fact, I have never heard of any other instance where someone was required to have one, as they are generally not admissible evidence in courts.

2. It is my understanding that the accuracy of polygraphs is mostly down to the skill of the person reading the device and giving the test (hence why they aren't usable as evidence).