Re: Double standards
In short (not agreeing/disagreeing - just as I see it):
BT + EE didn't reduce the number of mobile or fixed operators to any tangible extent (BT had minimal mobile presence, EE relatively insignificant in landlines) although there were concerns about BT having a dominant position in the provision of fixed links to MNO cell sites to EE's benefit EE under BT's ownership.
Three + O2: Tangibly reduces competition in the MNO market because it reduces the number of distinct MNOs from four to three. That doesn't take into account the network sharing deals MBNL and Cornerstone which have differing relationships with their constituent MNOs (Cornerstone is more site/Air Con/Power sharing; MBNL shares almost everything except spectrum).
BT could use its fixed line dominance to advance EE's market share/revenues &c but there remains three mobile operators (O2, Three, Vodafone) to compete with EE, the latter of which has substantial fixed/landline assets.