NickB

In exactly the same way as BA has agreements with lounge operators in various airports to provide lounge access. It does not follow from this that anybody traveling in any class on BA can access those lounges. A non-elite economy class traveler, for instance, will not normally have access to lounges regardless or the presence of agreements between lounge providers and BA.

BA has a policy of interlining when the flights are on a same ticket but not when they are on a separate ticket.

Interlining has a cost which BA is not willing to underwrite in the case of passengers on separate tickets outside OW. BA is not the only airline doing that even though it is true that a majority of IATA carriers still interline on separate tickets.

This is one aspect of the product offered by BA. You may dislike it but this is hardly news. It has been like that for ages. Other UK airlines have similar policies (even stricter in the case of bmi). IATA does not in any way require or even expect airlines to interline baggage on separate tickets. It is entirely at each carrier's discretion.

If you think that you can persuade BA to change, feel free to speak to them about it. IMO, this has just as much chance of happening as of David Cameron becoming a fervent Europhile but to each their own.

If not, and this is really deal breaker for you, feel free to fly another airline which has policies that suit your needs best.