Does a free app exist in the UK which can be installed on 2 android phones allowing you to make free data voice calls over an internet wifi connection for free without having to register a new account with the app installed? For example, with skype, you have to register a skype account before you can start using the application.
You can try viber. You only need to put your phone number.
Personally I don't like it because of the UI and their notification mechanism but its a matter of personal flavor.