Currently there is no change in BA policy and why should there be? the flight continues if it's safe to do so according to all the experts/manufacturers/regulators rules and performance calculations. If at any time you think it may become unsafe then you divert i.e. continue to LHR until you think (rightly or wrongly) you're not happy then go to an en-route. I'd like to stress the bit which says 'think'. This crew knew all the rules and probably spent most of the flight doing 'howgozit' fuel calculations, collecting alternate wx etc. There are rules to which you must abide but at the end of the day someone has to decide. Who better than the highly trained flight crew on the flight?
What if they had carried on to LHR, had a delay and landed below reserve fuel?
The FAA fine is political because there is no current regulation which prevents the crew doing exactly what they did. As usual it's all about interpretation of semantics which the lawyers get paid zillions to thrash out in a court.
And no, I'm not suggesting the lives of pax should ever be resolved by lawyers. An example is the BA diversion to Uralsk. No question, land at the nearest bit of concrete regardless of the cost implications for getting pax/luggage out. The Pilots are the best and only people to decide there and then what is best.
If you disagree then roll on the day when Pilots are no more and the flight is operated by a commitee of accounts with a remote control box.