The holiday pay you have ALREADY earned; it is based on days you already worked.
It looks like an unlawful deduction from your severance package. Did you get any letter when you were leaving? Fid you get any letter (of appointment) when you were joining?
Again, I believe these are items a citizens advice centre will be good on.
You could/should also consider a claim in the small claims courts for your holiday pay. The fee is usually small and you could claim the expense as part of your claim.
I had a massive sum (four figures) of holiday money withheld from me in a previous employment but as I had signed a legal document saying that all monies due had been discharged I disqualified myself from claim. A bitter lesson.
Are there any government contracts involved in that run, or to hospitals etc. They would not be happy to hear an uninsurable driver was delivering on their premises. There are major insurance implications for them.
1. Preliminary: You seem to have been "unfairly selected" for a redundancy. You should get advice/support on pursuing it.
2. Your holiday pay you are entitled to. Definitely pursue it.