"To be fair, Call Me Dave has not said TLS is to be banned, merely that the government will break into it on occasion when the home secretary has given personal permission"
ISTM that government agencies are already doing this, with or without the permission of the Home Secretary.
To my mind, this is, to some extent, more about access (or a right of access), to private keys. Quite what shape this discussion may take, should the Tories be re-elected, is uncertain - but personally I have a feeling that access to, and perhaps distribution of (or control of), keys within certain boundaries will hold a degree of focus within any such future discussion(s).
Until we know more about his dastardly plan, we'll just have to live content with the knowledge that number theorists, along with anyone with a desire to make use of prime numbers, will probably be labelled, vilified and persecuted as paedo-terrorists by god's 'right-hand man'*.
*Reference to Dave's Easter 2014 bash, where he said, "I'm just continuing God's work". (Alternatively, just read as 'wanker' - either works).