Valuables checked into the hold may well not be covered by your insurance policy and airlines tend to say they won't cover loss of valuables - their definition - either. Now this is not part of Montréal, other than anything more than about £1350 needs to be declared at check-in and a fee paid for it (there have been no recent reports of this even being possible, but formally it exists). So it maybe you could enforce your Montréal rights, but BA would probably say "it's against our conditions of carriage and had the traveller mentioned it, we would have refused to check it in to the hold". I guess that at some point the cost of these things isn't what it was, and no more odd than expensive clothes. It's not unusual for Middle East airlines to identify baggage during security screening and then push the problem back on to the passengers. Emirates has been known to ask UK airports to do this for them, Wythenshawe International actually will do so (and nowhere else in the UK as far as I can tell).