I think you need to get some proper legal advcie and not from ACAS (who aren't half as good as some people think). This could get very messy. In fact, it already is very messy. One thing that stands out is that you haven't been given notice of any kind, so technically you still work there unless you resign. But if you resign, then you won't be entitled to any redundancy pay - unless or until you are given notice or the company goes into administration, then obviously you aren't legally at risk or redundant, even though it's obvious that you are! If you see what I mean.
As for the unpaid wages, I am afraid that at best (unless they pay up which seems unlikely) it may take several months to get them. You would need to send a letter before action to asdvise them of the debt and your intention to take legal action if they don't pay you. Then you would need to make a claim to an employment tribunal for the money, which may take several months or longer to get heard - and if they can't pay then winning won't get you very far anyway. You'd have to try to get the money after winning, and when you couldn't, go back to the tribunal, at which point you may be able to make a claim to the Redundancy Payments Service (who deal with some unpaid wage issues, not just redundancy). But you may not be able to make a claim for redundancy because you have already got another job, and by doing that without having been given notice of redundancy, you may struggle to prove entitlement - I doubt you would be able to make a redundancy claim now given the circumstances, but that's a more complex area.
Not fair, I know, but I'm not sure that this is going to be worth the trouble, which is probably what they are betting on. That said, you'd obviously be stupid to not take up the new role in order to maybe have some entitlement to some money at some point in the future. I recognise that nobody can simply walk away from wages or other money they are owed easily, and I am definitely not suggesting that you do. But this won't be simple or straightforward, and it will take time and energy to pursue. Only you can decide where the balance is in that equation. If push came to shove I might suggest that you make a tribunal claim for the money and see what happens, but not throw good money after bad. If you get anything out of that, then that's good, but if you don't you aren't any worse off; and if it becomes too consuming for you you have the choice to walk away.
I hate bloody employers who do this sort of thing because they have known for ages that they wouldn't have enough money, and have been either stringing everyone along or burying their heads in the sand (or both) to see if it works out. And you are the one paying the price.