Dublin T2 is pleasant and efficient terminal, security is professional and quick without the negative and lazy attitude seen so often at LHR. Fast track is nice to have but has minimal saving at most times of the day. 3D scanners on being installed currently so no more liquids out of bag etc soon.

For US flights you get pre-clearance which is huge benefit, so no more G4S high vis on a power trip, no painful queues in the US to clear immigration/customs and need to recheck luggage in the US.

Aer Lingus service to LHR (and LGW) is more frequent, more punctual and more reliable than BA's service. The T2 LHR experience is pleasant and the EI lounge is directly above the gates, no messing with buses or inter terminal trains etc. If you can get yourself onto the A321NEO operated flights the Aer Space fare gets you a proper lie flat seat. As BA gold you can avail of the EI lounge while flying on EI metal to LHR/LGW.

Once you get status with EI, when departing Dublin just check in after 9pm the night before and theoretical seating Irish style will apply, just opt for automatic seat assignment for a seat in the front of the aircraft.

EI business to the US, free seat selection regardless of status, 3K and 5K are among the best J seats on any airline, free WiFi as well. AA also fly out limited destinations. Once you have do Dublin to US direct, you won't want to do LHR ever again.

Bristol is on a ATR72, it does the job. For Poland your choices are Ryanair or to fly with BA/LH/KL via LHR/FRA/AMS, fares likely to be cheaper than from London due heavy competition from Ryanair. T1 at Dublin can be a bit of a Zoo

Downside is EI is a low cost efficient carrier so expect basic or no lounge. But end of the day BA isn't exactly the worlds favorite airline and the experience in the back is pretty much the same, only difference is EI doesn't have free booze in Y on TATL.