BT Wholesale is the company responsible for providing and maintaining the network infrastructure for Internet Service Providers (ISPs) located in the United Kingdom. BT Wholesale's role is sometimes referred to as a "broadband wholesale carrier", "adsl wholesale carrier" or an "international adsl/broadband wholesale carrier".
ADSL is the technology which provides broadband over the landline copper telephone network of the United Kingdom. BT are a leading telecommunications company based in the UK, which until recently were state controlled. Until 1984, BT had a monopoly over the telecommunications network of the UK. In 1984, a Telecommunications Act was passed in the UK, which privatised BT and enabled private companies to purchase capacity from BT and sell telecommunications products.
Thus, the role of BT Wholesale becomes clearer: they are the company which provides ADSL capacity to private companies in the UK. BT Wholesale do not sign a contract with private individuals but with private companies, such as Internet Service Providers like Plusnet. This means that an individual cannot contact BT Wholesale to fix a hardware issue with their ADSL connection, because no contract exists between the two parties. The correct protocol is for the user to contact their ISP - who they do have a contract with - and the ISP will contact BT Wholesale to remedy the problem. It's a subtle and often confusing distinction of contracts and duties.