bmi has been selling slots for years - it once had around 18% of them, I think. With nobody else looking to buy at the moment, you're right - whilst it would be in the employees' interest for BA to move to buy fast, it's partly in BA's interest simply to pick up the slots at a rate they can grow into them, in the knowledge that nobody else is buying at the moment.
On the other hand, if they can pick up say 15 airbuses together with a cheap workforce for them direct from Lufthansa along with all the LHR slots, that may be in their interest as well. It just depends on the price.