The UK rolled probably rolled out free SMS texts later than the US; the US was one of the first markets that rolled it out. I don't know when whatsapp rolled out there but I'm guessing it pre dates unlimited SMS being ubiquitious.
iMessage has pretty much the same feature set as Whatsapp so US users don't really have a reason to migrate at this point.
iMessage has pretty much the same feature set as Whatsapp so US users don't really have a reason to migrate at this point.