When taking my disabled father to the USA I found the service at LHR to be excellent. Upon our return the helper at the gate took us all the way to the waiting taxi outside, he earned a decent tip and was worth every penny.

DFW was excellent, we were transferring and assisted all the way.

SFO was useless, security was horrendous, it took them 15 minutes to swab my Dad and his wheelchair and he became extremely uncomfortable, I ended up pushing my dad myself.

PHX and TPA were both very good.

LGW were useless, my fathers wheelchair was broken in the hold, which was brought to the door of the plane in pieces, we were loaned a wheelchair at the airport but left to find a way to get my father home without a wheelchair. The BA baggage desk basically said you are on your own.
The upside to this was that we went to a wheelchair specialist who said he would handle the claim for us and my father ended up with a new lightweight wheelchair.

All of the crews we encountered on BA, AA and UA were caring and helpful.