For your first seven days of sickness, employers can either use their own self-certificates as evidence of incapacity or accept confirmation from you, either verbally or in writing. It is important that you agree what type of evidence will be acceptable. You can get a self-certificate SC2 from your GP or you can complete it online at www.gov.uk/guidance/ask-your-employer-for-statutory-sick-pay .
Statutory Sick Pay (Medical Evidence) Regs 1985, reg 2(2)

After your first seven days of sickness, your employer can ask for supporting medical evidence. This will usually be in the form of a certificate signed by a healthcare professional: a ‘fit note’ (see below). However, medical evidence may be accepted from someone who is not a registered medical practitioner – eg an osteopath, acupuncturist or herbalist.
Statutory Sick Pay (Medical Evidence) Regs 1985, reg 2(1)(b)

It is up to your employer to decide whether to accept the evidence that you are not fit for wor