Here's
the EU regs on PSOs - start from page 6 of the PDF (Article 4) I think. Stansted, Gatwick and Heathrow are referred to as an "airport system" which may muddy the waters.
Since EI have admitted that SNN is (somewhat) profitable, the Commission may have trouble understanding why taxpayer dosh should simply increase EI's profit margin, especially when most PSOs are sub 50 seats rather than a 174 seat A320.
even then, EI must then acquire slots, take them from DUB or ORK, or cancel the BFS routes. If the latter, Big Ian will have a sense of humour failure because of the "treachery of Dublin"...
edit to note: since the PSO must be published in the official journal, any slotholding EU airline at LHR would theoretically be in line to bid for it - e.g. BA, bmi.