It’s common in the UK, but it isn’t the most used messaging app.

WhatsApp is more common in the UK

> I think iMessage is common in English speaking countries with high iPhone usage.

I can confirm that iMessage is extremely common in Australia. WhatsApp is very uncommon, outside of people with European (and maybe South American?) friends or family to keep in touch with. And given the country's demographics, I suspect there are far more people using WeChat to keep up with their overseas contacts in Aus than there are people using WhatsApp.

I use SMS/iMessage for most one-to-one messaging, but never for group chats. For groups, I use only WhatsApp and a little Signal. I'm in the UK.

I got into Telegram for group chat, I was surprised at the saturation of WhatsApp when I went overseas.