I would say that WhatsApp is probably the most commonly used chat app in the UK. But most people are also using at least one other chat service such as Facebook

I would say that WhatsApp is probably the most commonly used chat app in the UK. But most people are also using at least one other chat service such as Facebook Messenger or Instagram's messages. And of course due to the way WhatsApp uses phone numbers as an id, most people also have people's phone numbers and can fall back to SMS if need be.