The National Insurance system will allow you to Self Certificate for any period of illness where you are off work for up to 7 calander days. The clock resets if you return to work (or work from home) even for a few hours. During this first week your employer should accept your word that you are ill.
Many employers have their own self-certification forms, which the employee should complete and return to the employer. You do not need to see a doctor.
If you are having difficulty with your employer about this, the government rules on sick certification are a useful reference.
Some employers insist on a doctor’s note regardless. If we are asked to provide a doctor's note for a period of absence from work of under 7 days, we will charge a fee for this as a private sick certificate.