A smart meter HAN - Home Area Network uses the Zigbee smart home wireless networking standard, however it seems to use a different ‘profile’ to most other uses of Zigbee and this I believe prevents directly accessing it.
There are some dedicated devices that use the same ZigBee Smart Energy 2.0 profile and hence can in theory connect. (This may or may not be restricted by the energy and meter supplier.) An example is this one Product Page - EAGLE-200 - Automated Energy Management | Rainforest Automation
These connect to the HAN using Zigbee and then convert the data to a form that can be accessed over your home LAN. (RainForest have their own API for this.)
In fact this type of device has its own abbreviation which is CAD - Consumer Access Device. You may be familiar with some smart meter devices which provide associated device with an LCD display you can view in real time on your table, these are called IHD - In Home Display devices and also use the same HAN. In fact some IHD devices now also have CAD capabilities as well. See - Smarter energy - Green Energy Options : Green Energy Options
Issues to be aware of.
SMETS1 and SMETS2 are obviously different. Some IHD/CAD devices may only support SMETS1 others may support both.
I believe that whilst the most common radio frequency for Zigbee is 2.4GHz that some SMETS meters may use different frequencies e.g. 868MHz. This is because these alternate frequencies have a longer range and with meters often being hidden away in a cupboard or even outside the house the greater range can be vital. Obviously you need to be compatible in this respect as well.