I think you misunderstand and you seem to be using "iMessage" broader than the messenger service provided by Apple. SMS is not iMessage.
First, you don't need to pay anything for iMessage regardless of whether it's to domestic or international numbers. You only pay if it reverts back to SMS or if you send SMS text messages in the first place. Blue bubble free, green bubble not.
Second, of course WhatsApp is not a network standard in the UK or in large parts of Latin America. It's just very very widespread. So widespread that the basic assumption is that you will contact other people and will be contacted by other people through WhatsApp. I've seen it myself in Ecuador, where businesses put the WhatsApp logo on their ads, rather than a generic phone or text symbol. Of course you can text them, but they may not reply.
It's not quite like that here in the UK, but friends and colleagues will automatically use WhatsApp rather than anything else to get in touch with me. The network standard is irrelevant.