Since you had no sick days, you can put in zero and be done with it.
However, if you had sick days and / or simply don't want to answer out of principle or because you're uncomfortable to disclose such private information just leave it blank.
Chances are, they won't follow up.
I don't know UK law regarding this but it strikes me as a privacy and medical confidentiality issue and they probably have no legal basis in asking this information.
Should they inquire further you may straight out say, you prefer not to answer (which might let them conclude you had a lot).
You could also ask them directly for the reasoning why they want to know this sort of private information.
After all, it is between you and your former employer and short of pre-existing conditions not repetitive.
Frankly, I don't see this being any of their business.
Should they want to know of any medical condition that may cause sick days while you work for them, they should have asked for that instead. ( that is actually a much more valid question)
In a conversation where they are persistent and you simply don't want to give them that information attempt to deflect by saying something along the lines of
"I can't remember exactly, not many (if that's true)", or "Unless there would be a chronic health issue, I don't think past sick days would provide enough insight into future capabilities to work.", or "it doesn't seem to be relevant information that would warrant private inquiries"
make sure to say in any case or if they're irritated
"I'm not in the habit of calling in sick if I'm not."
because that is basically what they want, to see a pattern of behavior.
You may also answer in a light, slightly humorous form signaling "between the lines" that you don't recognize the validity / importance of the question nor intend to answer it.
If they're making a fuzz about this, you probably don't want to intern there anyways, so don't sweat it if they won't pick you over this.
...unless the internship is extremely good / prestigious or helpful to your academic career, then you might contemplate answering...
Get some free legal advice if you can, this is at least a learning experience for you on how to deal with future questions like these from prospective employers.