That the EU would look at PHL-LHR isn't too surprising. I expected the EU to look at the CLT-LHR slot pair that US obtained a while back which came from AA/BA as a concession to the EU when BA and AA were granted permission to implement their immunized joint business agreement.
Originally, that slot was intended for MIA-LHR, and DL tried that for a while before giving up. US applied for the slot and obtained permission to route it MIA-CLT-LHR (with a change of plane at CLT), essentially stripping MIA of the additional competition.
Of course, CLT has very little London O&D compared to PHL, so if the regulators ignore CLT, very few customers will be harmed by (presumably) higher monopoly fares. Still, the merger results in a reunification of that slot pair with AA/BA, airlines that were supposed to provide it to competitors from MIA.