"Nothing to do with that merger resulting in previously separate ownerships becoming combined use then?"
Indeed. This is exactly why BT/EE have a dominant position in the market. Nothing to do with spending big.
In the 4G Auction in 2012, EE bought a large chunk of spectrum that added to the already merged Orange and T-Mobile 2G/3G spectrum they already had (and re-factored quite a bit of their legacy 2G spectrum to 4G), and then BT themselves bought another chunk under the guise of a company called 'Niche Spectrum Ventures' with Ian Livingston at the time telling Ofcom they have 'No plans to become a national mobile operator'.
We all know how that ended up....
Problem with Ofcom at the moment is that the large telcos are pulling the strings and Sharon White is just being a puppet. Ofcom is meant to be on the side of the consumer, not on the side of the large telcos.