I haven't purchased any of the hardware supplied by BT for networking, but I have ditched their BT Voyager modem and am instead using a wireless modem router and a couple of wireless NIC. This is working a treat with no problems at all.
I bought the wireless router from www.ebuyer.com, and it was their own brand conforming to 802.11b standards, and 11b DLink wireless cards.
Thoroughly recommended for those on a very tight budget.
Just to add to Steve's excellent answer: Ethernet has a "top speed" of 100 Mbps (mega bits per second) whereas wireless is aprox 11 Mbps. For the internet this is plenty fast enough as you will only purchase a connection up to 2 Mbps (ADSL broadband is usually 0.5 Mbps). However, if you are transferring big files across your network you will certainly notice the slowness of a wireless connection compared to ethernet.