Get a couple of phone extensions from pound land. Join them together and place the modem next to the computer. Cost £2.
Connect a ADSL WiFi modem next to the BT phone line, were it enters the house. Add either an internal PCI WiFi card or external USB WiFi dongle. The result will be that the signal will be sent from the router, next to the BT socket, to the computer in any room in the house. Cost around £60. But you have no wires and you can easily pack it away when you next move.