A couple of points.
* where is the oversight to make sure what GCHQ are doing is proportional. "Who watches the watchers?"
and here is where I start to get properly worried about slippery slopes.
" it is enough that the warrant be necessary for the purpose of preventing or detecting serious crime, or (in some cases) preventing or mitigating death, injury or damage to a person’s physical or mental health."
So we go form on the one hand, "serious crime" which can have a nice definition that everyone can at least understand (if not agree) to "mitigating [.....] damage to a person’s [......] mental health" which can mean just about anything the state things is objectionable to just about anyone at just about any time !!.
A complete cop-out