The headline attached to this submission is sensationalist and misleading. GCHQ did not force CertiVox to shut down PrivateSky. CertiVox decided to close PrivateSky after they were served with a notice issued under section 49 of the Regulation of Investigatory Powers Act (RIPA) which required that they hand over the key(s) required to decrypt one (or more) of their customer's data.
The underlying legislation can be found here: http://www.legislation.gov.uk/ukpga/2000/23/contents
The relevant section is 49 but sections 53 (Failure to comply with a notice) and 54 (Tipping off) are also of particular interest.