Look at your contract of employment to see whether there any specific requirements, such as attending a medical exam, for receiving and continuing to receive sick pay.
Your written statement of employment particulars, which should be given to employees within one month of starting work, must specify the terms relating to sick pay, or refer to an easily accessible document which sets out those terms.
Usually the employer will ask employees to notify the employer early on the first day of absence. Often it allows self-certification for the first few days, before requiring medical evidence in the form of a 'fit note' from a GP.
For long-term sickness, or frequent short-term absences, the employer may, with your consent, seek a medical opinion from your GP or an occupational health (OH) specialist.
It is sensible to cooperate with your employer’s reasonable attempts to get expert advice about your medical condition. Otherwise, you risk your employer making decisions about sick pay and about your future employment without the benefit of medical input.