How to disable text messages and only allow for iMessages on iPhone?

I want to text a friend in the UK and I'm from the US. I know that if you use iMessages it's free but if Wi-Fi/3G is unavailable, it'll switch to text messages, right? So how do I make sure it never switches from iMessage to text message? I'm a new iPhone user so I don't know.

I don't think that's possible i think you could do it is not have a Sim card or any texting on your phone