Redundancy is one of the fair reasons for dismissal, but not the way he has done it. I cannot see how how he not in breach of company law if the company is continuing to trade when insolvent. It is possible to go into voluntary liquidation, or it is possible for a creditor to force insolvency, but it is not possible to say the business will be insolvent soon so the staff are being declared redundant now. And ACAS obvious know their stuff and merely confirmed this to you.
At the end of the day, I am pretty sure you will get your money eventually, but it is probably going to take a bit of time. On the evidence you have provided, you have been unfairly dismissed, but there is little point in taking an insolvent business to Employment Tribunal to prove the point and win, because the business won't have the money to pay anyway (unless you don't believe him).
I would write the company a very short letter along the lines of:
Dear Boss, on 16th May you told me that my job was terminated without notice, on the grounds that the company was insolvent. I note that the business appears to be still trading today, some 2 weeks later, and would be appear to be insolvent as you indicated on 16th May.
I further note that you have employed a new person in the business who would seem to be undertaking the work that I was employed to do prior to the 16th May. It would seem that my role as xyz is therefore not redundant.
I would be grateful if we could meet as soon as possible to enable you to fully explain this situation to me.
Then see what happens.
I therefore request that