Firstly, this is common, and you’re not the only one affected. The reason they gave you is BS. They just want you out, and this is how they operate currently.
Secondly, they will tell you to keep it to yourself. Don’t do it! Share it with your team and everyone you know. There isn’t any law preventing you from sharing it, your manager knows it, s/he put you at risk and s/he has shared it already with others. Don’t worry about the enhanced package, this has been agreed already with the Union, it has been published and they cannot take it away from you. Otherwise, it would be discrimination.
They will tell you don't need to come i
the office, come in every day! When your role is at risk, you still have a job and you can access the office. Come in, keep your temper, sit with you team, preferably next to your manager and read your favourite novel.
Most importantly, raise a grievance immediately. Again, they cannot withdraw your enhanced package, because if they do, it would be an act of retaliation.
The grievance must be reviewed by someone at the same level. The outcome will likely be that your grievance is deemed invalid. This is expected. Appeal it. Then, your grievance must be reviewed by someone one level above.
If they put at risk 100 colleagues in a round and all of them raise grievances which, by law, must be reviewed then all the managers, HR, and legal teams will be overwhelmed with dealing with grievances!
If they don't stick with the process consider seeking legal advice.
You have more power than you think! Check the portal and make yourself familiar with the grievance process and the redundancy package.
You're welcome ;)