I use EI to SNN a good deal. As an AA PLT, you can use the Premier check-in lines and lounge at LHR and DUB. But, be sure get to LHR early because security is CHAOS on a good day (unless you can use the fast track). The walk to the IRL gates through the tin cans takes a solid 10 mins. Also that section of LHR rivals the old terminal (I forget the name) at DTW as world's ugliest airport terminal. An experience you won't forget. The lounge at LHR is rather plain but compared to the surroundings on the other side of the door it is a palace.