As I understand annual leave, you are entitled to accrue up to 8 weeks leave - i.e. 2 years - and can then be required to take it. Agree with you that the way the matter was put to you was poor. One reason to belong to a union is that, when you are being told "do it or else" there is an expert on your side that you can ask. The 'HR' department work for the bos, so tell you what the boss wants them to, just as a real estate agent works for the vendor, not the purchaser - although both will tell you they are fair and impartial.
Flemo, the approach to sick you advocate is inequitable.
It is an entitlement, not a right, i.e. you are entitled to use it if sick, but don't have a right to bit if you are not. If you are sick then take the time off, if you aren't go to work.
Say, for example, I work for years and am well, so have lots of sick leave accrued when I retire, while you / your spouse have a bad run with health, so you use yours as sick / family leave. We both retire at the same time.
I have never understood why, in those circumstances, I should get paid a 'bonus' for having been healthy, while you get nothing.
Yes, I understand that, in some industries, employees approaching retirement take off 'months' of accrued sick leave before retirement. Well, if the employer believes this to be bogus they should challenge it. Would only take one case to stop the rot.