Why does NatWest take payments early?

That's fairly normal for most banks. Debits are entered on the last working day before a payment is due. Payments due on Mondays are usually debited on the previous Friday.

People often used to withdraw money from an ATM over the weekend forgetting that wouldn't leave enough money in the account for payments due on Monday, which then failed to go through. When failure to make a payment on time involved some kind of penalty they would then blame the bank.

I don't think it necessarily involves any loss of interest. I believe banking transactions have two dates associated with them, a "ledger date" and a "value date". The ledger date is what gets shown on your statement, but interest is calculated based on the value date.

Thanks Chris, understood.