Once a PO line has been partially processed, the VAT Code is locked on those lines. To edit the VAT Code, use one of these workarounds:
Set the GBP Price to zero for the locked line, then enter a new line for the same quantity and original GBP Price, then enter the correct VAT Code for that line.
Close the locked lines using Close Lines, then enter new lines for the same quantity and original GBP Price, then enter the correct VAT Code for those lines.
Close the entire PO (or lines) and enter a standalone Purchase Ledger Invoice for the value of the lines.