You do not need a sick note for the first 7 days. You self-certify for this period.
To get a form click on this link https://www.hmrc.gov.uk/forms/sc2.pdf
Please also note that it is not possible to give backdated certificates. The doctor may be able to if there is proof that you were ill e.g. in hospital having an operation and for a reasonable time, but in case of doubt you should contact the GP and not take a chance.