You can have change your SAIL address in just a few hours if you notify Companies House by filing form AD02 online via WebFiling. There is no charge. Sometimes it may take up to 24 hours for the address info to be updated.
You can also report the change of address free of charge online through your company formation agent. This is normally quicker than WebFiling and there is no need to fill out a form - you simply add the address details and submit the information to Companies House electronically.