I am in a roughly similar situation myself - although in my case, the part of my redundancy figure up to the £30k tax exempt limit was NOT included in my P45, but the part above £30k WAS included.
To the best of my knowledge, (and according to the HMRC web chat representative I spoke to) any part of a redundancy figure appearing in the P45/P60 is a mistake on the part of the employer.
Based on that web chat, when I submit my own tax return (waiting on some other documentation), I am planning to deduct the redundancy figure from my P45 value, add it back in under the relevant "Other income" boxes, and write an explanation in the "other information" box citing the web chat advice I received.
By the way, since you've had redundancy figures included in a P45, you might want to double check your employer didn't over-charge you National Insurance contributions - the portion of a redundancy payment above £30k is subject to Class 1A employer's NICs but NOT Class 1 employee's NICs, somewhat unusually.
Regarding your comment about not being able to separate tax on the P45 into itemized components - happily you don't have to, as there is a question in the "Other income" section "Have you left 'Tax taken off' blank because the tax is included in your Employment pages?" which lets you declare all the tax taken off in the Employment pages and none in "Other income".
As if all this complication wasn't enough... it turns out there can be an actual difference in how much tax there is to pay, after moving the additional redundancy payment above £30k from the P45 figure on the employment pages of the tax return, to the "Other income". This is because of the statutory order in which difference kinds of income are taxed. In my case, I had a small amount of dividend income, which is taxed after regular employment income, but before additional redundancy lump sums. Since dividends are taxed at different rates, and have their own nil rate allowance, but still contribute to the assignment of income to basic/higher/additional rate bands, dividend income can push the additional redundancy payment into a higher tax band than it would have been at, had it been treated as ordinary employment income. (I include this in case it helps understand why the total tax due might change, just by moving the redundancy payment from one box to another.)
And then... it turns out HMRC's online self assessment filing web pages have a bug in the "View your full calculation" feature ... it looks like someone made a mistake in the conditions for when to display what parts of the calculation, and the "Redundancy, lump sums, compensation etc." section is missing out from the full calculation breakdown (even though its sums are still added to the bottom line) ... unless you have dividend income that exceeds the dividend allowance (£2000 in 2022-23)! That *really* did not help, with understanding the "order of taxation" thing, nor with just wanting to confirm the sums added up right.
