I fail to see how this can work on paper...
3 and EE have a RAN share (MBNL) for the 3G part of each network. They have recently-ish agreed to allow a 4G site (not RAN) share.
O2 and VF have a RAN share (Cornerstone) for the respective 2G and 3G network (dont know if this extends to 4G)
For either 3 or O2 to to divest either of those agreements will significantly impact the respective network, aka a really really bad idea!!
Access to O2's 2G spectrum will benefits 3 users and access to the 3 3G network will benefit O2 users for data. This would only be a shared PLMN which is not as a smooth as it could be, hence Orange & T-Mobile moving from a shared PLMN to a single PLMN code for EE and both carriers underneath.
They have no benefits to derive from contiguous spectrum, neither operator has a fixed carrier, with VF buying C and W and the BT/EE marriage they will be left to the side for backhaul.
Paul