Unless things have changed, once passed back to BA itself , rather than dealing with K2, it will apply 10% per annum depreciation - hard, imo, to argue against that being a valid level

That you may not have travelled doesn't impact on depreciation

The protections ( Montreal convention for most international travel ) only make the airline liable for based on value of loss. Same as someone has an accident in a 5 year old car - the person at fault is only liable for the value of car after depreciaiton - doesn't provide a way to get a free brand new car