All of the data which forms the basis of this website is obtained from public, freely available sources. The core data which forms the postcode list itself is supplied by the Office of National Statistics (ONS), and augmented by other statistical datasets such as those supplied by Ordnance Survey, Ofcom, the Land Registry and Ofsted.
Each of those datasets have their own limitations. In particular, the geographic coordinates of postcodes supplied by different providers do not necessarily match. In most cases, this site uses the ONS coordinates. Other data, such as broadband speeds, is only updated anually and may, therefore, be significantly out of date at certain times.
A major limitation is that this site does not have access to official full address data. The full Postcode Address File (PAF) published by Royal Mail is very expensive to licence (far beyond the resources of a simple, advertising supported site such as this one) and comes with usage restrictions that would, in any case, make it unsuitable. All address-related data, therefore (including street names associated with postcodes) is derived from other published sources (such as the Land Registry). This is reliable enough for the purposes of this website, but you should not use this site as a replacement for the PAF or for services provided by Royal Mail's authorised resellers.