If I resigned and the split was amicable I'd be expected to work my notice.

If I resigned and the split was acrimonious I'd want early release from my contract. I'd not want to be there, they'd not want me to be there and they would not want to pay me.

If I was let go then I'd be expected to be paid in lieu of notice.

The reality is that the cost of a months salary is much smaller than the benefit of a single employee.