It's not an emergency is it? It's not right to take an emergency appointment that someone else might need urgently. Speak to the receptionist, if you definitely need a certificate the doctor might speak to you on the 'phone and issue one. You can self-certificate for a week as well.
They're now called fit notes, by the way.
A fit note must be signed by a doctor but you don't always need to see your GP in person to get one.
It depends on: why you're off work sick
Whether your GP needs to assess you face-to-face
if you've been in hospital