Not sure what BA can realistically sell passengers that World Duty Free cannot. Not to mention the issue of loading it and storing it.
The duty rate on a litre of vodka is somewhere between £11 and £14, so even Amazon struggle to compete with duty free. A lot of alcohol sold in duty free shops is duty paid at the moment but I expect that to quickly change. Duty free shops have a massive opportunity with EU traffic becoming entitled for duty free sales.