You need to get the hacked APK to obtain it and get the free $10, and then it's basically impossible to top it up. (Need a US address)
Until it officially launches in the UK, it's not really worth worrying about.
You also need to be rooted and push the .apk file to system/apps
It's not impossible to top at all you have to use a third party service to do it though, and risky as google may block use of the hacked version at anytime and you'd lose your credit.