In the wake of successful prosecutions of three British men who conspired to bring down trans-Atlantic aircraft, the former Director of Public Prosecutions in the UK, Ken Macdonald, said that intercepted emails and phone calls were critical.
According to the AP article, Britain is one of the few countries in the world to bar the use of evidence from intercepted personal phone calls, e-mails, letters and faxes.
the best evidence you can have, people convicting themselves out of their own mouths.