You need to go back to the employer and asked them to issue an amended P 45.
If they don’t pay you for the 20 days, take them to the Small Claims Court or the Employment Tribunal.
You only have 3 months to take them to the tribunal so don’t delay.
Can I clarify anything for you?
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It’s not a rule. It’s a contractual requirement. They cannot give you a P 45 to end earlier than you were dismissed. They have to pay to that date also including holiday pay.