If you are off work due to being unwell, you do not need to see a doctor immedicately, You can "self-certify" for the first 7 days.
All information regarding sicknotes can be found here - Taking sick leave - GOV.UK
If you have been off for more than 7 days and need to request a sick note, you can -
- fill out a sick note request form
- phone or visit the surgery from 8am to 6pm