Using iMessage between London and USA?

Hi, I'm currently spending 2 months in London and I have an iPhone 4S. So far I've been using an app called What'sApp to text my friends back home but since I have an iPhone I'm wondering if iMessages would work. Basically I want to know if I'm connected to Wi-Fi can I send a message to someone back in the US (who has an iPhone too) for free? Does the other person have to be connected to Wi-Fi as well or will they still be able to text and respond if they are using 3/4G.

Both phones will need to be connected to Wi-Fi. You need to be VERY SURE that you are chatting with an iPhone user connected to Wi-Fi (bubble in blue, and not green), otherwise that can get expensive. They can't be connected to 3/4G while they're talking with you through iMessage, since 3/4G is managed by cell phone carriers, and that is obviously going to be very expensive if they're using it to communicate overseas.

I'd just stick with WhatsApp. Otherwise you can risk making a very costly mistake.

Can you be more specific please?

Make sure you are using Wi-Fi, otherwise you are connecting via the mobile 3g/4g network and it will be expensive. Also look at Viber, Skype and other VOIP ( voice over internet protocols) systems.