I am a freelance DN and charge £16ph in my area, there are 2 agencies in the area one charges the practices £15ph and pays the nurse £9.50ph plus travel expenses and the other charges £16ph and pays the nurse £10ph no travel but supplies uniform.
You also need to realise that working freelance/agency you are not guaranteed work although in my eyes you only need to work 3 days a week to get paid the same as what you would in general practice. You are also entitled to paid holiday leave.
Hope this helps