From the datasheet: "This device does not provide for direct verification of code memory contents. Instead, this device provides a 32-bit CRC result on either a sector or the entire user code space."
The other day, I did a little more digging around and found an older (serial) programming spec for the LPC900 series, in which one of the items noted in the document revision history was "removed READ command". So it may be possible that this is an MCU with write-only memory
On the other hand, what the datasheet/official documentation says, and what's actually possible, may not be identical.