The way BA & K2 act when they are dealing with damaged luggage is disgraceful. I went through something similar not so long ago.

As per others' advice, just take the money and buy what you want in the sales. You will be much better off than paying them near full price for something they will almost certainly be getting at trade price. You just have to accept that BA/K2 will seek any opportunity to devalue your existing luggage (depreciating it for every year and then further cutting the cash offer they make to you).

Then post some negative comments on BAs social media to make sure everyone is aware how happy they are to screw even their most loyal customers these days!