In about a week or so, I am considering either PHX-LHR (BA) connecting to LHR-GVA (BA) and a return of GVA-LHR (BA) connecting to LHR-DFW-PHX (AA)...

or

PHX-LAX-LHR (AA) connecting to LHR-GVA (BA) and a return of GVA-LHR (BA) connecting to LHR-DFW-PHX (AA)...

Is there any difference on the eastbound trip arriving AA vs BA? I have not transited LHR in probably 10 years. Will I need to clear any sort of security or passport control for the transit? Any thoughts on the quality of service on AA from LAX or BA from PHX going eastbound?

I will be in Y on an AA milage ticket and have never flown BA long haul, only within Europe on the PRG-LHR route when I lived in Prague.

It looks like today's BA 1527 (PHX-LHR) diverted to DFW.