Yes, WhatsApp is very popular in the UK and worldwide. It is originally from the US, in fact it was founded by former Yahoo workers in California in 2009 (the founders are Jan Khoum and Brian Acton), but it quickly went global. Today WhatsApp deals with up to 27 billion messages every day and has 250 million users around the world.
From the UK download page you can get an idea of the variety of apps and services available.
There have been some changes lately to the way WhatsApp is available to UK users. Until recently there was a small one-off fee to use it on your iPhone, but this has been replaced by a subscription, though still less than £1 per year. The idea of the subscription is that you pay these to avoid the company having to generate revenue by having lots of advertising on you iPhone, which is annoying to users. You can find details here.