Kees,
Standard ATP will take the LOWEST of all the atp quantities available. So, if you have Storage location ATP quantity of 5, batch ATP quantity of 5, and plant ATP quantity of 0, ATP will confirm 0 (or in your case 'lead time'); since plant is the lowest of the three.
In standard, you cannot disable the plant level check without disabling ATP entirely.
Refer to http://service.sap.com/sap/support/notes/547118 question 6.
Best Regards,
DB49