You could get an ADSL Router/Modem combo. It connects to your phoneline like the USB Modem, but provides Ethernet ports for your computers.
www.adslguide.org.uk has a list of Routers and links to where you can purchase them. Network Address Translation should hide the multiple IPs from BT. However, it may make gaming a problem as it normally needs port forwarding.
You could also try another ADSL provider that allows multiple IPs, get ther router and use port forwarding. I have just signed up with Zen and they have provided 8 free IP addresses.