To be precise: You will need a Deed Poll, which is a piece of paper with certain words on it, and it needs to be signed and witnessed, but you can make one yourself and you dont need an official body or lawyer to do it.
“I [old name] of [your address] have given up my name [old name] and have adopted for all purposes the name [new name].
“Signed as a deed on [date] as [old name] and [new name] in the presence of [witness 1 name] of [witness 1 address], and [witness 2 name] of [witness 2 address].
“I [old name] of [your address] have given up my name [old name] and have adopted for all purposes the name [new name].
“Signed as a deed on [date] as [old name] and [new name] in the presence of [witness 1 name] of [witness 1 address], and [witness 2 name] of [witness 2 address].
“[your new signature], [your old signature]
“[witness 1 signature], [witness 2 signature]”