I have seen graphs in BA inhouse fleet magazines where departure fuel over flight plan minimum and arrival fuel over flight planned arrival fuel were compared.
The articles always make interesting reading but serve to better inform rather than coerce.
The alleged jumbo story may have an element of truth. If it was BKK then Utapao is sometimes used as the fuel alternate if weather is suitable. I find the alleged figure of 2 tonnes remaining a little hard to believe and I have actually seen or heard nothing about such a diversion although it may be true. I will try and find out.