I didn't realise they did broadband via the TVDrive.
You do need a router to be able to connect more than one computer to the broadband.
You'll only be getting 1 IP address from the modem in your TVDrive.
A wireless access point can't share a single-IP connection, it's just like the wireless equivalent of a hub.
To share a normal single IP connection, you need a router (or use Internet Connection Sharing, which is free, but annoying, & requires having one "server" computer on all the time whenever you want to connect with another).
Routers are quite cheap now, even wireless ones. Linksys are a popular make here.
You'd need a wireless cable/DSL router, which would act as a NAT Router (to share the connection) + ethernet switch (for wired devices) + wireless access point.
If you get your own, make sure you don't get an ADSL modem/router.
If you continue using the STB for broadband, then the router would simply plug into the STB, while your computers & Xbox connect to the router via wires or wireless.
If you get Telewest to install a standalone cable modem, then the router would connect to that instead of the STB.
There are various threads with router recommendations.