Commissioner for Oaths fees can vary
If you need to make an affidavit or get certified copied of documents you will be charged for the service in most circumstances.
The standard fee applicable to solicitors for taking an oath is £5 but one way or another some lawyers charge more. It can be an amount for the oath and a separate amount for an ancillary service. There are also providers that you can find on the web that charge much more. Prices as high as £29.50 for swearing an affidavit or making a certified copy.
The simple answer is to shop around for a provider. For a one off single document this might not be much of an issue but if you gave a significant number of documents the difference in cost could be substantial.
Find out more on http://commissionerforoaths.co.uk