If your US phone is Android and you are willing to leave it plugged in on your dresser back in the US, there are programs like:

http://www.androidzoom.com/android_a...rder_hikh.html

that will forward your SMS messages to another number such as a Google Voice number. Get an Android phone for the UK, buy a data package, and use Google Voice to text the US.

Truphone offers a US phone number with a SIM that works relatively inexpensively in the UK. You could use that as well. Whatsapp is great, but it is like BBM and doesn't use text messages. If you are just texting a girl friend or a parent with a smart phone, that would be fine, but it wouldn't work for your entire social circle.