Anyone can access and use the AddressBase products, but how you pay for it is entirely dependant on whether you work in the public sector or the private sector.
For those in the public sector there is the Public Sector Mapping Agreement (PSMA), which in essence means that the Department of Communities and Local Government (DCLG) has paid for you to be able to use the data.
In order to get copies of AddressBase and many of the Ordnance Survey mapping products available via the PSMA, a public sector body must become a signatory. It is possible to sign up for the PSMA from the Ordnance Survey website.
Those in the private sector can license any of the AddressBase products directly from the Ordnance Survey, but software will be required in order to utilise it.