As you have already learnt, the best piece of advice for the return trip is to start at the right airport.
What class of travel are you returning in? BFS is a bit spartan, even in the lounge that your flight uses. However it is not a bad place to hang around in. Not too crowded at the time your flight departs.
When you are heading from LHR-BHD, it is a long walk from the main T1 area to where the Aer Lingus flights depart. We don't call it the "Green mile" for nothing
Allow 10 minutes at least. If you are on any sort of business ticket it might be helpful to check whether they have any sort of code share that would let you into the Gold Circle Lounge operated by EI at the end of your walk to the gates. It is a nice lounge.
Getting to BFS from downtown Belfast depends on time of day, but not less than 40 mins.