Haven't run across an NB1 in quite a while here. Heck, we haven't even had a 'post code' question for any unit of that vintage in quite some time. The NB1 is what we call a Nav2 device, a couple of generations back now.
The first two digits of the serial number will tell us if it has "Live" traffic ability or not. "DR" units do not, while "DP" units have Live traffic. That won't change our answer to your immediate question, but puts things in the focus for future. Also, you should be aware that this device has only 1GB of internal memory. It is quite likely that TomTom isn't supporting it with any new maps -- not even just UK&ROI which would actually fit a unit that 'small'.
First, are you able to enter the full post code, or just the first characters of the post code? If you can only enter the first 3 or so, then we need you to check your unit to assure that the post code file is actually included with the map set. Look for a folder in the root of your device called "zip". Within that, you should find Great_Britain.postal, Great_Britain.tlv and Great_BritainPostal.ver.