This is starting to sound like a 'divide and conquer' tactic. Really the best thing for me to do is to refer you to the whole of the Government's own advice on redundancy here - you may have already looked at the whole thing as I referred you to the 'trial job alternative' bit earlier.
https://www.gov.uk/redundant-your-rights/overview
You can work out your statutory redundancy and notice period from this and other sites - it depends on your age and length of employment. That, of course, is the statutory amount and it is very common for decent companies to pay more (often very much more) than these minimums.
What you and the employer are currently involved in is the 'consultation' stage. The consultation should be individual to you, and this is where the 'alternative job' scenario should be addressed.
I think it is perfectly reasonable (though I am unsure of the legal obligation by the employer) that at the end of this stage you are given a letter that says something like:
1) you are under notice of redundancy and if redundancy should occur the date of termination will be X (at least your notice period - one week per full year of employment up to a maximum of 12)
2) you and us have agreed that there may be alternative role and you have agreed to undertake a trial of 4 weeks
3) if at the end of this trial you reject the role, clause 1) will apply
4) if at the end of four weeks you take the job, there new T&Cs will be [etc etc - including the new hours).
That's the way I think they should do it, but you could check what ACAS helpline has to say.
In other words, I don't think they can mess around denying you the right to know what redundancy payment would apply until the end of the trial. The consultation should determine what offer is on the table to you.
Finally bear in mind that the notice period IS likely to be bound up in your period under which you trial the new job. It is worth finding out whether, if you take the package on offer now (no trial job), they are asking you work out your notice, or whether they would be paying you off. The reason why this make a difference to you is that you will not pay tax or NI on money paid in lieu of notice. Of course (you are proably know this also) you don't pay tax or NI on redundancy payments either (up to £30k, in case there are any pedants monitoring my answers).