The "premium" afternoon tea exists to serve BA's purpose than the wishes of their "premium" pax.
A few BA stale sandwiches cost pennies, the fillings are cheap and to my taste, rubbish. That's across both CE and CW. It achieves one goal, its very cheap. They already did this with Galleries First (aka Oasis of Calm) by replacing a lot of counter space with more cheap sandwiches. Of course, sandwiches don't have to be cheap, but BA are not interested in using quality ingredients to give a tasty sandwich full of tasty ingredients that's filling. They're more bread than anything else, the tiniest slither of processed ham etc. Afternoon tea's are popular in the hospitality trade because you can buy a loaf of processed white (BA style) bread for £0.70. Given the portion sizes, BA could cater entire batches of flights with one Morrisons super soft loaf, I recall they are two for £1 or £1.10.
If pax really were crying out for it, they could just load a few and adjust the demand over time for those who desperately want that crap but that would cost more money and the intention is to get away with catering the flights as cheaply as possible which is why even the Afternoon Tea has been cut.
It seems they do the budget in a cut something here, add something here style, but overall its balanced out from the crap to the decent option. We don't have the seafood meal anymore, which was a way to avoid the AT, and that was a budget decision because otherwise it would still exist. Full size bottles of wine/champagne for some flights, small bottles for others. My last AT flight I found the scone is pre-prepped which saves on the cream, the jam and the likely food waste of over-catering the scones as well as cutting the cake but the long routes got better and more food.