It is not necessary to have a Doctors certificate until you have been absent continuously from work for 7 days including weekends. After 3 days you must obtain a self-certificate(SC2) from your employer. If your illness continues for more than 7 days then under the NHS your GP can issue an official statement (Med 3) "fit to work certificate" to confirm your illness.
Your employer may require you to complete a self-certificate (SC2) which is available from Reception, your employer, the Post Office or the DSS or access the form online.
Read more information on when a Fit / Sick note is required