The problem is if the redundancy is due to happen at the end of the month, you can understand why they want to consult with you as soon as possible, and quite rightly so.
Also, from what you say, they did give you two weeks' notice of the original meeting. That's not unreasonable really on the face of it, especially given the tight timescale they were on.
You are entitled to be accompanied by a colleague or trade union official, however that doesn't mean you can delay the meeting unreasonably because one specific person is only available on one day, a day after the consultation period has ended. Again, quite a lot of notice was given for you to give your supporter, and it could well be reasonable of them to say that if this particular supporter isn't available to help you even with two weeks' notice, you need to choose someone else.
I think given they are also happy to either do the meeting as a conference call or even to come to your home, and given that the potential redundancy is supposed to be the end of this month, it would be difficult to claim they are not making every effort to accommodate your needs tbh.
Why is your supporting person so unavailable? Can they really not rejig things a bit to be there for you this week? Specifying one day, beyond the consultation period, that they can do the meeting is really very restrictive of them.
Is there anyone else you could have with you? You mention that you are struggling to get legal advice - why is that? It might be better to wait to get that until after you've had the meeting anyway - rather than talk about potential issues that might arise, go to a solicitor with actual facts and details of your employers actual actions, rather than potential ones.