The Vigor 2800, 3100 and 2910 now support the addition of an optional 3.5G USB modem, providing high-speed Internet connectivity over the latest generation of cellular networks. Several UK networks already provide HSDPA data at speeds of 1.8Mb/s up to 7.2Mb/s (depending on network and modem), meaning that you can get broadband speed without a fixed line.
You might also want to take a look at this. Alternatively, with custom firmware like openwrt, you could probably turn any router with a usb slot into what you need. I'm going to have a play with this sort of thing, since I should be getting an O2 usb stick like you soon.