yes. any of the counter staff. However - check that you have the up to date form. It used to be necessary to swear an affidavit (special proceedure affidavit) but the form has been changed (D80) and now has a ''statemetn of truth'' which you sign, but do not need to swear or have witnessed.
If you do need to swear an affidavit then a solicitor will charge you £5 for the affidavit and £2 for each exhibit.