When you use What's App, you send texts and make calls using the internet.
If you're in the UK and sending texts abroad, that's fine as long as you're using your paid-for allowance, or if you're using free Wi-Fi.
If you're abroad, then you need to make sure that you're either using free Wi-Fi, or you've got a package that includes using the internet abroad .... if you use the internet abroad and you pay by the MB, it could get quite expensive .... so check your internet allowance etc :)