For periods of illness of up-to 1 week it is possible to self-certify by completing an SC2 form. If your illness lasts longer than this, you will need to see one of the doctors to discuss completion of medical certificate. The department of work and pensions website provides detailed information about the processes of sickness certification. Printable copies of the self-certification form you can use in the first 7 days of illness can be found on the HM revenue and customs website (although most employers should provide these):
Department of work and pensions: sick pay info
HM revenue and customs: downloadable self-certification form.