Actually this does make sense at a political level but not in the way people might think.
The problem for the UK government would be the pressure from the NSA and US DoD to stop further leaks from the UK. Cameron’s problem is dealing with the US pressure, stopping the leaks is Obama’s problem and Clapper’s.
So from the very narrow standpoint of Cameron’s problems, destroying the drives makes perfect sense even though it obviously won’t stop further leaks from the copies in Brazil or Germany. Incidentally, we are now up to a million stolen documents.
Destroying the drives also prevents the US from pressuring the UK government to attempt to subpoena the drives as evidence so that they can find out which documents Snowden took. The NSA still doesn’t know what they lost. If the Guardian had kept them, there would have been pressure on HMG to get them back.
The UK has a national interest in protecting the special relationship with the US. In particular the unique intelligence sharing relationship. But the UK does not have a national interest in help the US avoid looking stupid.
Cameron stopped the hunt by shooting the fox.
