Here are my 2 cents.
Perhaps the DEN, PHX, SAN flight change to LHR may have nothing to do with an upcoming Open Skies agreement. Perhaps, British Airways made a request with the US and UK government to give them an exemption for these 3 cities.
Perhaps the US DOT and UK saw that these 3 cities have limited European flights, and to better promote them, they decided to allow these 3 cities to move to LHR because of the better connection opportunities than LGW.
Notice how BA has made no announcement about the DFW and ATL flights moving to LHR. Possibly because these cities already have substantial European connections, so they were not given exemptions for these cities.
SAN's only Europe connection is the British Airways flight. LGW is very limited when it comes to connections, and it is getting more and more limited each year as British Airways transfers routes and flights to LHR.
DEN and PHX are also similar, each with only British Airways and Lufthansa providing Europe flights.
In Dallas, you have American with many destinations in Europe, plus the couple European carriers. In Atlanta, you have Delta with many destinations in Europe, plus the several European carriers.
This is a strong possibility and it makes sense.
Anyways, let's start talking about the benefits if these changes really happen.
This is a good article in the Rocky Mountain News:
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran