In a former life, I spent a good deal of time on the 888 and 889 as was. I miss it so.
Short answer to your question is probably not in the near future.
There's only so much demand BEG-LON. And it's limited to those Serb nationals that can get a UK visa. Transfer pax heading west across the Atlantic have better options through FRA/MUC/FCO/CDG/AMS. And JU now has a direct A330 service most days BEG-JFK.
O&D business is acommodated adequately, it seems, by JU and Wizzair.
It's a shame. One of Europe's great capital cities and not on the BA route network.