EI is slightly better off if they keep then, but they will have to deal with FR, hence I am suggesting that they sell the LHR slots, deal with the FR problem and still make money on LON-IRE...
Just how slight is the benefit to keeping them? I reckon it's a lot more than slight. In addition, selling LHR slots does not deal with the FR problem, in fact it would add to the already considerable cash pile EI have and make it even more of an attractive take over target by people/companies interested in nothing but that cash pile.
Selling the slots to BA would mean EI having no control over them, and who is to say that BA wouldn't reduce services from DUB/ORK/SNN/BHD over time especially if as you say extra slots would mean them better able to compete on other routes?
Finally, just because BA doesn't hold the same % of LHR slots as AF does at CDG does not mean they are short of slots. Don't they already lease some out to other airlines?