Just to add its not the mobile phone signal that you should check but the companies HSDPA signal. This is the "super quick" reception you need from the local transmitter to make the broadband "quick". Not all transmitters are upgraded yet. My advice would be to check your postcode on the companies website (3, Tmobile etc) and if you need help head into one of the companies stores.
I have a datacard with "3" and find the reception nationwide very good. In fact i get a regular 1meg connection while on the road which is quicker than i can get at home.
I must add that i think mobile broadband is fantastic and enables me to work anywhere in the country as if im at my desk - and its so cheap £10 a month !!
Smala01