If you're off work sick for 7 days or less, your employer should not ask for medical evidence that you've been ill. Instead they can ask you to confirm that you've been ill. You can do this by filling in a form yourself when you return to work.
If you are off work sick for more than 7 days then you will require a Fit Note and should contact your GP. A Fit Note will be provided for you by your hospital doctor if you are an inpatient or are receiving treatment in hospital and this should also cover any forward period that they consider you will be unfit for work.