Unfortunately it is not possible to change your username within this forum, mainly because your username not only applies to your forum name, but also your giffgaff account, and is used to recognise you also (for example affiliate link). Also, it has been mentioned that Lithium (Forum software) doesn't have the ability for staff to do this for you.
The only thing I can advise is for you to create a new account, this is if you desperately want to change it. As if you make a new account, you'd have a new number and would loose your current points and credit. Of course you can use both accounts, but I'd recommend you just stay with your current account, it would be much easier than going through many complicated things just for a username.