From what I read, the police arrived within 2-5 minutes after the attack started. The only way to pre-empt it would be to have information about the terrorist's plans beforehand. Encryption on phones is widely available to the average person, and this makes it very difficult to stop terrorists from planning attacks.
In the grand scheme of things, terrorist attacks do not kill very many people. However, they brutalise public discourse. Every time this happens it draws us closer to authoritarianism: loss of privacy, mass internment, ethnic cleansing. The conversation after attacks seems to be becoming more and more sarcastic and bitter on one hand, and counter-establishment/authoritarian on the other. For example, photos of Sadiq Khan telling the population that these attacks are 'part and parcel' of living in a major city; and the mockery of people saying "we don't yet know if this is a muslim".
I'm pro-free-speech and pro-encryption, but I am not sure whether grinning and bearing it is really an effective weapon against terrorist activity, considering its ratcheting effects on public opinion. So, if we don't ban encryption, what are we going to do?