Poorly timed opportunism. Police have said the attacker acted alone, so he wasn't using encrypted comms to talk to anyone.
They can track his purchases via his debit card, his movements via CCTV + cell tower records, intercept his emails... but there's one bit of his digital life that's inaccessible and we're "going dark?"
We are burning bright with data. More data does not necessarily mean less terrorism.
The English might be better served by posting some armed officers in high value areas. The French do this at major train stations and tourist spots like the Eiffel Tower. This doesn't stop terrorism, but vastly reduces the body count.
Frankly, I think it's laughable that countries which resisted the Nazis will let 10 people dying make them consider rolling back civil liberties.
For most of history, governments have not had the ability to easily monitor the communications of their citizens. Widely available, user friendly encryption tools are just returning us to normal. Well, except for the massive trail of metadata everyone now leaves.
However, can't they already find out who the message was sent to? Whatsapp obviously has to have that information, and it appears they will give it to law enforcement:
https://www.forbes.com/sites/thomasbrewster/2017/01/22/whats...
I'm not sure that knowing the contents of that message will really help more than knowing the person who it was sent to.
It seems that they had this guy on their radar a few years ago, but didn't think he was worth keeping an eye on, so even if they could decrypt whatsapp messages it wouldn't really have helped them.
http://www.aljazeera.com/news/2017/03/uk-police-arrests-west...