So those that say we only really have 2 networks are wrong. Plenty of people nowadays are saying EE and 3 signal will mirror one another and o2 and vodafone will also. Not true 
Exactly. The fact that the two MBNL 3G networks have a lot of sharing going on doesn't mean they are the same, and for 4G they're both a lot different. The two Cornerstone networks are quite different for both 3G and 4G, and its really just a physical site share.
It's significantly more complex than people like to make out.
For 4G, they share masts, backhaul etc - very sensible as the latter will be expensive as usage is drive upwards, but they install the antennas, equipment and so on independently - effectively think of it as them sharing the "connectivity" to the site - whereas they connect up whatever they want after that. There is thus in the 4G share potential for "different heights" although there is more to the theory on height vs coverage than has been made out. This is less cost effective, but means they have more freedom.
For 3G, they share base stations and antennas. - so the theory that three are located below at a lower output is total rubbish because they have the same ones - and they share spectrum, and so on as part of that - saves lots more money but means both networks effectively have to work with each other very closely.