The recent success of EI is built on Trans Atlantic connections, a huge chunk of which is from the UK regions. IAG, particularly BA must have been looking on for the last 2 years or so with annoyance. Putting a stop to that before it becomes a true threat combined with 24 LHR slot pairs a day pays for itself in the long run from their point of view
To some extent that is in BA/IAGs own hands - people have been shafted too often by domestics being dropped at the whiff of problems at Heathrow meaning the connecting TA is missed westbound, or an uncomfortable wait at LHR or slow coach journey as a "reward" for using London Airways coming East. Add in US pre-clearance, and EI offer quite a few advantages for the GB regions.
That said, the offer may be more a strategic attempt to warn off the airline that may not be mentioned on Pprune....