This is odd. If the FA put the bag in the overhead compartment above you, did you not see anyone going into the locker? It is also odd that the flight attendants deny that they placed the bag in the compartment overhead, when you say they did and that you have witnesses.
The problem is this. You don't know who stole your items and can't prove who did it. Therefore, your only hope of recovery would be to prove negligence by BA. It is unlikely that you would be able to claim that BA was negligent in placing the item in the overhead locker. It is in the Terms and Conditions that you follow any reasonable request of the FA and in this case the FA made a judgement that your bag needed to be in the overhead compartment for safety. You have no chance of proving otherwise, as it is a judgement call. If your exit seat was a bulkhead seat, then you do indeed have to put it in the overhead compartment, as the space under your own seat is in fact for use by the passenger sitting behind you. If you were in a exit seat which was not a bulkhead seat, you are still trying to argue against the judgement of an experienced FA. I think any court unlikely to find in your favour in those circumstances, even if you, through your witnesses, are able to prove they did put the bag in the locker. You would however have the satisfaction of proving the FA a liar.
However, you could argue that the failure of the cabin crew and pilot to call the police before disembarkation was a negligent failure which resulted in any possibility to recover your stolen items. You could claim that BA impeded the possibility of catching the thief and recovering your property by not calling the police to the aircraft before disembarkation, despite your request that they do so, and are therefore liable. The only reason for the failure of the staff to call police to the aircraft was for operational efficiency. I think you could argue that this was possibly unreasonable. However, that in itself is a judgement call. For example, if you were in crowded store and someone picked your pocket, could you reasonably request the store lock the doors and restrain all the customers and staff from leaving until the police arrived? Would this be reasonable?
If you wanted to, you could file a claim in Small Claims Court claiming negligence by BA. You may find BA willing to settle rather than go through the hassle of trying to defend their actions, especially if you have indepedent witnesses. You can file a claim cheaply in the UK using the online small claims court procedure. This will force BA to respond. You can file a claim here:
https://www.moneyclaim.gov.uk/web/mcol/welcome