This is the response from Admin at Tom Tom. Looks like the 520 doesn't feature post codes at all. I don't know what use a Tom Tom is without using post codes.

This is Nikhilesh again from TomTom Customer Care Team following up with you. Thank you for your response.

I understand you are unable to search for post code on your GO 520 device but as the post code doesn't have street name your device is not able to locate the address and want to know what to do in this case.

I have checked and found that this is how the TomTom device works as it requires Street name to navigate to and as there is no Street name to it you are unable to search for the address on device.