Family deals aren't as common over here, Vodafone are the only people I've ever seen advertising anything similar. Most kids over here have pre-pay mobile phones that you top-up, and adults have contract (monthly-billed) phones.
What network your friends are on isn't quite as important as it used to be - most bundles now include "any network" minutes (i.e. free calls to people on any network), but remember, unless it states "any network" it will mean landline or same-network calls.
Coverage isn't as much of an issue as in the US, some networks will have slightly better coverage than others, but you'll almost always get a signal unless you're out in the deepest, darkest sticks.
Personally, I use
www.o2.co.uk as their online tarrif is very reasonable, and I've never had any problems with the network - I'm sure others would beg to differ and offer their own suggestions.
The main mobile companies are:
www.vodafone.co.ukwww.o2.co.ukwww.orange.co.ukwww.t-mobile.co.ukwww.virginmobile.comwww.three.co.uk