why you can't just connect a USB broadband modem directly to your computer? As you have only one machine this would be the simplest/cheapest option, assuming that the computer is a PC, and not a laptop.
If you have a laptop, or wish to move the PC around the flat however, you'll need a wireless router, and a wireless network asdapter for the computer. I tend to favour Netgear routers at the moment, but I have also used (and been very happy with) Belkin hardware. I have no personal experience of the BT Voyager 2100, but to be honest, a wireless router is a wireless router, is a wireless router - they all do much the same job.