aren't United renting slots from Aer Lingus at Heathrow at the moment
No, in fact I believe it is EI that are renting (in) slots from another carrier, mostly to cover the SNN-LHR service...and so would have something of an issue when/if that carrier decided to take the slots back for its own use.
Doing a 'land grab' on possible LHR slots coming out of the BD-BA merger is just about the only way EI is going to be able to retain flexibility to maintain/grow its LHR operations going forward without shelling out significant amounts of money to buy slots.
Remember they would only have to operate EDI/ABZ etc for 3 years (not an insignificant amount of time, I appreciate) before the slots become theirs to do with exactly as they wise (ie. use for other routes if desired).