Since redundancy payments are normally determined as x number of weeks pay, then I would think that applying a tax rate equivalent to that which would have occurred if it was paid out over that (x number of weeks) time period rather than as a lump sum would be a fairer way of doing it.
In the end it will all be balanced with either a rebate or demand at the end of the tax year, so the ping pong argument over evasion/avoidance is irrelevant (there’s a feud thread if you want to continue it). But it doesn’t address the need for cashflow now, rather than a refund post March 2021. Perhaps you could apply for a lower rate now in the knowledge it may incur a bill next March?