Although I have no experience of Macs, I'm quite sure it is possible to connect wirelessly to a router with one and therefore to Telewest. Mac-compatible hardware must conform to the industry standard protocols for wireless networking, 802.11b and 802.11g, and use the same TCP/IP protocols for interent access.
It is possible that Telewest don't offer technical support to set it up, but that doesn't mean that it cannot be done. I recently bought a Buffalo Air Station G54 wireless router (under £50) which I am using with Telewest cable, connecting with Windows ("wirelessly" and "wired") and with SuSE Linux ("wired").
You can probably pick up a suitable wireless router more cheaply yourself than getting one from Telewest - just be sure to get one that DOESN'T have a built-in ADSL modem as those are only suitable for use with ADSL connections.