Hi BM- yes, I accept that it's a breach but, unlike your example of a building dispute, it seems to me that in an employment case it is a breach to which there can be a tacit agreement between the parties.
If instead of tax-free PILON the employer said "okay, there's no work, I'll continue to pay you monthly for the next three months and you can come 'work' from home or come in to the office if you want", then tax would be payable on the earnings.
I'm not sure why HMRC doesn't press this issue a bit more.