Can I buy a broadband router from BT .... without a corresponding broadband service?
Indeed you can.
Have a look at http://www.shop.bt.com/category/networking,routers-and-switches,routers/11304
I have a choice of 2 BT hubs to use for my Broadband connection: a 2700HGV and a 2701HGV. Does anyone know the key differences and if given a choice which would be the preferred model?