BA will never buy some, all or none of EI, or any slots at LHR. BA no longer has that level of strategic decision making power. BA is not an independent airline in its own right, it is part of a multinational holding company. Strategic decisions such as acquisition of airlines, aircraft or large numbers of slots are made in Madrid, not London.
LHR is no longer, necessarily, the be all and end all of operations these days. MAD is a large hub with equally modern facilities and room to grow. IAG could, if it wished, siphon much more connecting traffic through MAD and fill more LHR flights with O&D. Indeed this makes a lot more sense than trying to develop an LGW hub that some keep suggesting.
Selling the premier route in the short haul network makes absolutely zero sense from EI's point of view. Even when times were really bad, LHR was always the last route to be cut. I could almost guarentee that the last EI flight ever returning to DUB would be from LHR, the only way these slots would be sold is in liquidation. The LHR operation is at the very core of EI.