All the good advice has pretty much been said really.

Really depends on how much data you need and for what use, as for speed, as said a really good signal is needed (ideally from a number of Base Station Sites, rather than just rely on one), even then your in the hands of the Network Data Management, which most Networks use now, and can see stripped down to a bare minimum, so questionable as to whether 4G its a direct replacement for home broadband.
If your just trying to save some dosh on the Broadband front, some providers will supply you a basic package without the phone side, fairly sure Virgin do or did a no phone line required deal.
Although Im not a fan of EE, of late there doing there best to convince me otherwise, they are coming on stream very well now, and when there 4G speeds drop to 3G (HSDPA+) you can still get decent speeds on it, Ive seen speeds of around 15mb on 3g.HSDPA when I did some tests. They also have some deals up there sleeves for existing Customers that aren't normally advertised ie, 15GB data Sim for 20 notes a month or a simple Home Broadband for £15. Might be worth looking into if anyone in the household uses EE, Orange or T Mobile.
