Most educational establishments have a sickness absence policy stating that you can self certify for a short period without needing a doctor’s note.
The Sickness Absence Policy for the University of Liverpool states that a self certificate is valid for absences of up to 5 days (with a few exceptions). Most universities will have a similar policy.
If you do need a doctors note, this is considered as non-NHS work and therefore there will be a charge. The current charge for a note is £10. Letters will cost £15 or £25 depending on the level of detail required. (Prices correct as of July 2014, subject to change).
Please note that from August 2014 the University of Liverpool will no longer be covering this charge for their students, and the student will therefore be required to pay this directly to the practice if a note is needed.
The University of Liverpool has published guidance on the supporting evidence required for mitigating circumstance applications. This can be found here.