Read the Official History of MI5
If you read the Official History of the Security Services "The Defence of the Realm: The Authorized History of MI5" by Christopher Andrew, you realise that communication interception and reading (of content not just metadata) has been central to almost every success that they will admit in public. The exceptions being mostly major defectors.
I suspect the prospect of strong encryption by default is deeply worrying to them. But as plenty of people have posted, they have only themselves to blame.