You can call yourself whatever you want so long as it is not to impersonate someone else. As you will probably need to register and prove the change of name, (passport/rental agreements/bank accounts, etc.) it is better to arrange a simple document which confirms the change and the date it takes effect. See a solicitor.
and I meant to add, unless youv'e got a good laser printer and quality paper better taking their �24 option where they will post the document out rather than email to you.
Goog luck
A Deed Poll isn't strictly a necessary step under English law either. Under law, your surname is whatever name you are known by, so if everyone calls you X , a deed poll whereby you say you want to be Y makes no difference if the world still calls you X. However, a deed poll is a useful piece of evidence to show that you have the settled intention to be known by the name you state.That's why people use them, to show that they are serious.