If you want to change your name officially and legally through the Court the Court charge is £102, if you do it yourself.
http://hmctsformfinder.justice.gov.uk/courtfinder/forms/loc019-eng.pdf
You don't have to that - but paying a third party is money wasted. You need only print out the form I have linked to, get it properly witnessed and send copies to the relevant agencies - bank accounts, passport, driving licence, utilities, mortgage company, landlord, credit card companies ....... all of which you have to do if you pay a third party.