I think the reason why BA do not offer such extras is because of how popular WT+ is for paying customers and for the puropse of oversold Y cabin.
It's rarely empty.
Having LHR as their hub and the number of destinations they fly too, BA have a very strong advantage over other airlines, where convenience will most of the time out weighs a stronger product.
RE: VS Prem Economy - they dont have the connecting flights and number of other LH destinations as BA, so a large share of the Prem Econ market goes to BA, even if VS offer added perks.