EI could probably add an A330 to a morning Dublin/Heathrow rotation and drop one and hence free up a rotation on the route without a drop in seats, however there are a few issues. 1. There is 2 classes of service on the A330 that is not suitable as an ongoing offer on a Dublin/London service. Secondly, if weather hampers flights on the Atlantic ie delays by weather or simply issues around the schedule it will have an impact on crucial early morning flights. Management are not in favour of this option.
Furthermore While BA may have slots for swap lease etc. These you will find are not usually for early peak slots as we want. They will be at a price more than EI may want to pay and the code share arrangement will have no bearing on this price.
According to EI website BFS LHR will be x3 daily for next summer.... doesnt sound to me like they want more slots....?