Your employer cannot ask you to provide medical evidence that you are sick for the first seven days of illness.
During this period they may ask you to fill in a self-certificate of their own design or form SC2.
You can download a copy of form SC2 from the HM Revenue and Customs (HMRC) website using the following link.
If you are sick for more than seven days, your employer can ask you to give them some medical evidence from your doctor.
This is used to support payment of Statutory Sick Pay.