So presumably, if they had got there first 3/O2 would have been just fine and Orange/Tmobile->BT would have been blocked.
To my mind EE should never have been allowed, that made something altogether too big, by contrast I have real issues with BT (or maybe Openreach splitting off cleanly and owning a cell network with BT and others being an MVNO on top of that) owning a Cell network, it makes sense in terms of product offerings and many consumers like getting everything in one place.
However 3 networks on one piddly island that loves to use mobiles should not have a real problem being competitive. If it proves to be, I guess we could just sell some of their spectrum to a new entrant.