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It's very difficult to in the first place.
When I briefly worked for one of the security services (which is why I disappeared from the forum for a bit) all phones and smartwatches were left outside the secure area, the room I worked in had all documents locked in a safe before leaving for the night. 'Secret' marked documents were printed on coloured paper to make it obvious they were secret, and were not allowed to leave the room, the computers we were working on were on a network not connected to the internet (which made front-end dev very difficult indeed) and the internet-connected ones had no access to anything work-related.
Seriously though, what happens to Home Office or MoD employees who use WhatsApp or personal email for government business? What happens to a MI5 analyst who mishandles documents and sends to the wrong person?
Two friends who work for the MoD have to surrender their phones at front gate security before being allowed into their Whitehall office. Why is it that MPs are allowed to chat shit on personal phones?
The gap between what is acceptable in Parliament and what is acceptable in Whitehall has never been larger.