You can self-certificate yourself for the first 6 days of any absence from work through illness. Self-certification forms are available from either your employer or any DSS office. They are particularly appropriate for minor illnesses which do not necessitate a visit to your GP.
It is also possible in some circumstances for your doctors to issue a fit note without using up an appointment, ie. if you are already under care of the hospital. The receptionist will be able to discuss this with you; this may save both you and the doctor time. Hospital doctors and nurses can, where appropriate, also issue fit notes.