120.4, I agree that the captain was merely doing what he thought was best.
There would be many other considerations which he would have to take into account which we are not party to. These would include the weather conditions at his diversion and the amount of fuel remaining.
I can see that theoretically 27L was above his cat 2 limits, but it does seem that a good policy would be (wind permitting) to use the runway with the lower limits as the main landing runway in order to avoid possible go arounds and diversions.
Is the noise lobby possibly the reason for not so doing?
It does seem strange to me that BA do not accept the promulgated classification of the runway.