In my case, I tried several times to get BA to sort this out and improve the case settlement. To no avail. They simply say that K2 Global is their appointed expert and they abide by their valuations. I had been travelling First, as a BA Gold. That makes no difference. YouFirst wouldn't get involved either, they referred me back to BA Customer Services, who in turn refer back to K2 Global. Maybe policy has changed in the last 12 months, but I doubt contacting BA Customer Relations will get the OP anywhere.
it will get things progressed to a cash settlement. The level that the settlement is BA is likely to stick with whatever K2 say your bag is worth Ans to be fair, when I had an issue I did do some research to my bag and what they were selling for second hand and there were actually being generous. i suspect the realoty is we underestimate how quickly luggage devalues.
With K2, I do think it also really depends which agent you get. The one I got last time was fabulous and have kept his name (sadly FT rules prohibit posting it on here).