Likely they are trying to clarify their position to you... or if they are wrong (and going against their contract), trying to scare you into submission. Check your contract and if that isn't there then I suggest you go to a Union to get some support in escalating your concern.

Here in the US that would probably be considered retaliation. I'm not sure about across the pond, but here in the US you would probably have a case of you had things in writing.

But yeah, almost certainly. Always have a paper trail. HR doesn't exist to protect you, they are there to protect the company.

If you simply don't go to work, they can do that. But in case of medical things, the doctor will write a paper saying you can't work for some days, and therefore they can't take off your holiday pay. In any case, they should have called you and explained what exactly happened and the exact values (and days) involved. If there's a law saying they can do that, they must show it to you.

...likely...