You do not require a doctor's sickness certificate for any illness lasting seven days or less. Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website
For any illness lasting longer than seven days you will usually need to speak to the GP in order that they can issue a Fit Note (MED3) and for any subsequent continuation certificates.
'Signing off' certificates are not required.
If your employer insists on a certificate for an absence of less than seven days, this can only be provided as a private certificate for which a charge will be made.