You could (make that *should*) have another phone socket fitted in your bedroom - ask BT to do this when they sort out broadband. A more complicated and expensive way to do this is to get a wireless modem/router (or even a wireless access point) when you get broadband & also a wireless PCI card/network adaptor for your PC. This chunk of hardware would sit at your phone socket broadcasting to your PC upstairs.
Personally, I'd stick with the extension cable/2nd phone socket as an alternative would be pretty pricey in comparison.