The user manual says this:
|Return Code||CMD_SUCCESS followed by four 32-bit words of a unique serial number in ASCII format. The word sent at the lowest address is sent first|
But the code in the LPC5411x package is this:
/* Read the unique ID */
uint32_t command, result;
command = IAP_READ_UID_CMD;
I'm not sure is that's just buggy code that just keeps floating around, but it clearly won't work. Digging around in the forums I see that this exist in several LPCOpen packages.