I have Orange, GF has O2.
Hers works in here in Lincolnshire, mine doesn't
Mine works in Shropshire, hers doesn't.
Which area are you most concerned with? Get the most detailed coverage map for that area you can find.
Slightly OT, correct me if I'm wrong but a 999 call can hijack any network, whichever has the best signal rather than whichever your contract is with? The principal is the same when roaming abroad, why can't the regulators get the networks to do something like that for regular calls? They can make them bring down the cost of international calls, complete waste of time, only helps those who can afford foreign holidays and no doubt the networks will put up the prices for the rest of us to compensate.