Yes, but you can set up security on your router such as WEP, so you need to enter a passkey to be able to connect to the network and therefore use the Internet, and obviously you'd set that passkey so only you will know it
Edit: do you mean, could his router connect to your telewest modem without you knowing? No, the modem has to be physically plugged into the router via Cat5e cable. Only the computers connect wirelessly to the router, not the modem lol :P