Telewest do (or at least used to) only allow access from specific MAC addresses. It used to automatically register the MAC of the first piece of equipment connected to the cable modem, and from there on out, only that MAC can connect through the modem. In addition, it used to be the case that you could ring them up and register extra MAC addresses with them, thus allowing multiple devices to connect, so you could give this a try.
Probably the easiest option though, is to plug your router into the modem, then plug the laptop (the one which can connect) into the router using the wired connection (don't plug any other computers into the router to start with), then if you go to your routers setup page from the laptop, there is usually an option called "Clone MAC address" or something similar, if you activate this, the router should detect your laptops MAC address, and then alter its own MAC address to be identical to the laptops (you may need to disable any firewall sorftware running on the laptop).
After rebooting all the equipment concerned (including the cable box), the router should be able to connect through the cable modem. If not then there must be some other issue (such as that your router is not set up to automatically obtain an IP or DNS). Forget about wireless set up untill you can first get the router to connect, and your laptop to connect through a wired connection to the router, Once that is done, then you can set up the wireless.