Yes, you may be able to go back to work after being signed off if you can provide the adequate fit note or statement of fitness saying that you are not ready and prepared to get back to work. In other cases, if you get better before the assumed date and want to get back before the end date on your fit note you can discuss you going back to work with your employer.  Some workplaces may not be able to agree to an early return, which would mean that you might have to stay off work until the end date of your fit note.