I never really understood why BT flogged off Cellent in the first place. Maybe they thought that the mobile pone wouldn't take off as much as it did ? 
I was with BT Cellnet . . . . . and I've never changed. Still the same number except for when they added the "7" a few years ago.
Simple......they spent FAR to much money on the 3G licenses and needed to get cash and offload an asset.
Unlike TB I am of the opinion that O2 would be a much better option than EE as they have considerably more base stations in much better locations. Then to move the network forward its only hardware upgrades rather than having to go through issues of site purchase and planning for masts as you would with EE.
Also a lot of the sites were originally the Cellnet ones and are close to BT assets already.
Assuming they can resolve a key technical issue, I'm not convinced its interested in either.
However, if they do feel the need to acquire a network, its again no secret that BT's mobile network, BT Mobile (an MVNO currently), are in the process of dumping Vodafone and pushing all its customers onto the EE network.