The wireless broadband from some companies is a marketing tool, where you pay more and they supply the kit. Unfortunately some BT routers ONLY work with BT, so if you buy them you're stuck on BT.
You can buy an ordinary broadband package from any company and buy you own Wireless router & cards seperately.
Tiscali for example do a good value 2Mbps broadband & unlimited Uk telephone calls for £19.99 (+£11 BT rental). Then buy adsl wireless router (Belkin F5d7632UK or Netgear GD834G for £60 + cards for PC/laptops)