The US and UK's "special relationship" extended to allowing US security services keep masses of personal data harvested about UK citizens, including our email addresses and mobile numbers, as the UK's spying laws were altered by the then government to keep the NSA happy.
Documents seen by Channel 4 News suggest that at some point between 2004 and 2007, the NSA was given permission to keep incidental data harvested from its mass surveillance schemes. Previously, an agreement between the UK and US stated that such collateral spying data harvested without explicit permission of each government had to be binned.
The reason given for the policy change is that due to the evolving nature of the digital world, keeping such data may help agencies piece together links between the various mobile numbers and email addresses in use. [Channel 4 News]