Re: "...for example to stop terrorists..."
Also indicates to the reader that there is a better than average chance that you are a bigot.
I'm sure GCHQ could knock up an AI to detect that. Most of the signatories already have detect-o-bots and other algorithms to make sure all our personal data is scrutinised, analysed and packaged for themselves and their valued partners. Value probably being $$ multiplied by the number of personal information categories shared. Then there's de-platforming people who might upset their advertisers, or executives. Such irony when the usual suspects complain about privacy infringement when our data is of such value to them.
And as for Brexit.. who's idea was all the data retention stuff imposed on SPs? At least post-Brexit, there'd be just one statutory butt to kick, who's purpose is to collect electronic data.
But being global, and most of the usual suspects being American, the US can already require most of this from US companies, or just data transiting US services.. And depending on politics, may mean sharing is reduced.. But that's also GCHQ's challenge, ie unless it's purely domestic terrorism, endpoints may well be completely outside UK jurisdiction.. Like most webmail.
Then there's the practicality. So GCHQ gets a copy of all message keys when they're created, so it can decrypt messages. So all you need to do is bypass that process, or use multiple layers of encryption.. Which miscreants are known to do as they've become ever more aware of security services capabilities. Which means the serious & organised criminals stay relatively secure.