Employer asking for a sick note. England.

Hi, not sure if this is relevant for this sub but basically I went in for a shift this morning and I have got another one later in the evening, however I have suddenly been sick and have got diarrhoea since this morning. I just don’t feel well enough to do a busy bar shift so I called in saying I feel unwell and have been sick, the manager is asking me for a doctor’s note telling me that if it is food borne illness you’re going to have to go to the doctor and get diagnosed and get a fit note and be monitored.

I thought for under 7 days self certification is good enough? Is he just being a dick and should I call back and say that I do not have to provide one or the doctor said I do not need to provide one?