Try CAB first to seek 30 minutes free solicitor time.
If everyone worked their notice out it doesn't help - I was working off the angle that if no-one did (work notice), then your hubyy was being discriminated against by the employer's refusal to declare the redundancy - and if it all happened at once it might have helped also. It didn't - people left in drabs and drabs from what you say.
Statutory redundancy amounts are tiny so the real 'prize' is getting the 12 weeks notice money.
I guess for the same reason (employer believes he may get caught by the unfair dismissal legislation) the employer won't state that your husband is dismissed by reason of capability - the (fair) reason typically used when an employee cannot work again. Once again, notice has to be paid - so there is no incentive.
Now I see why you are left with merely trying to extract holiday pay. Good luck - worth going through exploration process but I don't think I can help further.