Apparently, Thomson offer Firmware customization kits to 3rd-party's to add branded logos, TR-069 Management platforms, and their Setup Wizard adds "Select a service" of PlusNet ISP or Other. They're not locked to PlusNet (etc) but are probably locked from reverting to Thomson Firmware, or eBay would be oblivious to suppliers.
You can see their changes here:
http://www.plus.net/support/broadband/hardware/t585v7.shtmlhttp://www.madasafish.com/support/internet/gettingconnected/setup/thomson585/install_585v7winnew.shtml