Everyone will have their own preference for lounges, but I would personally not go via LHR for T3 lounges, especially business lounges (you don't have a status in your profile so assume you don't have OW Emerald to get into the F lounges).

I'd also likely go the IB route back via MAD to avoid LHR. It's a slightly longer flight in euro-biz but I don't love LHR either. Having said that my last EI flight to LHR actually used the TATL config A321 with a proper business cabin, I don't know if that's the norm, but was a surprise.