I disagree with ABetaDad about not telling your employer why you don't want to sign. This is a consultation period. They are proposing a change, which you are within your rights to refuse, but they can potentially force it through and part of that process is giving you the opportunity to raise any concerns you have. If you don't raise concerns but just refuse to sign, you are not being reasonable.
If you look at the link I posted, what happens when you refuse is that the employer can decide whether they want to try to force it through or not. They can dismiss you and re-employ you immediately on the new terms, and it would then be up to you to claim unfair dismissal. If they have made every effort to consult with you, and listen to your concerns, and if there is a genuine business reason for the change, you may struggle to win an unfair dismissal case. If you claim unfair dismissal and you haven't told your employer what your concerns were and given them every opportunity to address them, your case will be even weaker.
What they are doing is trying to demonstrate that they are doing their best to consult and listen to concerns. They are asking you to raise concerns by Monday to give them an opportunity to address those concerns. Should those concerns not be addressed to your satisfaction by a week later, then you would not sign.
I would suggest you speak to as many of the others involved as possible and find out their thoughts. Acting together is always better. If you have concerns about the proposal, raise them as requested by Monday - I would do it in writing. Just say that at present you are not planning to consent to the changes proposed because you are concerned about x,y,z way that it will have a negative effect on you. Say that, should your concerns not be addressed, you will not be signing the new contract.