All communications of public office holders should be made public. Uncluding their private ones. Once this is done, we are okay with this law. Else, it's just not right. These people have the power to hide their own criminal acts.
Practically speaking unless Meta are willing to remove WhatsApp from the appstore for UK IPs and lay off all 5,000 staff at Kings Cross on principle, do they have any leverage?
looking at breadth and depth of surveillance circa 2013 (prism/xkeyscore), plus meta's track record i'm sceptical they don't already have access to it all