Maybe it doesn’t come across that well, but “you just have to laugh” is generally used in situations where you’re acknowledging that things are so beyond rubbish that it’s now comical.
Yes, I understand that, and also T8191's comment about having to laugh otherwise you'd cry.
But the OP ended his opening post with
Originally Posted by BOH
Caused a fair bit of amusement......brilliant
which does suggest that he (and possibly other passengers) did find it amusing rather than irritating.
If this were a one-off service failure on a world-class airline then I would undoubtedly also find the idea of having the jam and the cream but no scone to put them on quite amusing. But this is BA, which fails to deliver every single day - mostly small things, but things that add up to a significant overall degradation of the product - and seems to not care about putting any of it right. Afternoon tea features prominently in BA's marketing and, the whole CE offering now being quite meagre, the scone would be a significant part of the promised meal.
It was only a scone, but I would have complained (despite knowing that I would receive only an insincere template apology, if even that, in return). If passengers don't complain then BA gets away with it yet again.