GPs are not obliged to provide their patients with a sick note (Fit Note) for the first 7 days or less.
Your employer should have a supply of self certification forms (SC1/SC2), which you should complete on return to work. A link to a printable version has been provided below for your convenience.
If your employer requests a medical certificate for less than 7 days illness this would require an appointment with a GP and would incur a private fee. These fees are payable at the time of consultation but might not be reimbursed by your employer.
The practice charges for other non-NHS services including fitness to travel and fitness to exercise certification. A fee will also be charged for any investigations or tests required in order to provide such certificates.
Non-NHS Charges to Patients 2018
SC2 Form