There are 9 British companies who supply private security services.
I was put through to the British Association of Private Security Companies, and spoke to their CEO, Andy Bearpark. He told me there was "zero" chance that British companies were involved in Libya. I have no reason to disbelieve him.
In our conversation, interestingly, he said that "mercenary" activity is in fact illegal. South Africa has substantial legislation to forbid it, but UK law is vastly outdated.