Would never accept the ISP cheap or free USB modem offering with a BB connection. USB modems are prone to drop connection if power supply through USB line is not sufficiently stable or strong. This can be caused by lots of other USB devices pulling power or a PSU that is not quite up to the overall task.
Would recommend shelling out a few quid more for a standalone ethernet dsl modem, preferably with a router built in. Router will contain a hardware firewall and also allows for easy networking and internet connection sharing should you want that (either now or later)