There is a bit of a game here, in that airlines are required to minimise their losses and have a suspicion that claimants over-egg their inventory of lost items. It is not a requirement to have a receipt to prove loss, and ultimately someone could take an airline to court to establish their claim, something airlines try to avoid. BA, on the whole, has not got a bad reputation in this area, mostly claims are met in full. There have been three historic posts in this BA forum where someone has - in my view - taken the mick, two were paid, one was not.

My advice would be after packing your bag, unpack it again after taking everything out over a table top, bed or floor and taking a photo. Then repack. This actually doesn't take long, the repack process will often give a better result anyway. This doesn't have to be every time one flies, since usually most people, most of the time, take very similar items. Not only does it help with claims - mainly in terms of not forgetting something in the claim - but it also helps with a future trip to prevent something from being forgotten. I do this from time to time as a HBO traveller, that isn't a complete immunity here, if you end up on a smaller aircraft or even a full Embraer 195, but for checked luggage it is a quick and easy way to keep on top of matters.