With this in mind, if I were the OP, I would be respectful of BA's rules and also in the interest of not potentially putting the crew in a somewhat-awkward spot of having to refuse, I would not ask or otherwise try to obtain alcoholic drink on a BA flight for my travelling companion under the age of 18.

To me, the issue of the right and wrong of not serving alcohol to under-18s is not really relevant, and what is relevant to me is wanting to respect the "house rule" of BA and not potentially create an issue for the crew regardless of the 'law', but that's just my personal position and I suspect many others will take a different position.