If they now take the employer to court, how are they going to prove they were ill without medical evidence?
Remember that a tribunal isn’t a court; the rules of evidence are very different. You don’t have to
prove things so much as
convince the chairman that you are being truthful.
So, if you walk in and say “I was sick” then unless the employer presents a video of you at Wimbledon or some other compelling statement that convinces the chair you were not ill, a pattern to sickness for example, then in most cases as far as the chair is concerned, ill you were.
That has at least been my experience, usually, which is why I avoid (as an employer) IT’s where I can.