At the time I couldn't find any option on Windows phone to be able to edit or forward a text message once it was sent. Having got home I checked online and found the answer.
It is possible to forward an SMS text message on Windows phone but you have to press/hold down on the message you want to forward and a new menu will appear that gives you the option to forward it.
This seems to be one of the big differences between Windows phone and iPhone - a lot of the context menus require you to press/hold down on the screen in order to display the new menu. On the iPhone many of these menuy links are visible eg forwarding a message has a tick box next to the SMS text that when selected allowing forwarding.
|How to Forward an SMS Text Message on Windows Phone|