First, the BP issue while a convenience has little to do with OW. Depending on the specific connection point, one may obtain onward BP's at the origin and sometimes at a connections/transfer desk (MAD by way of example) or at the gate or kiosks which exist for this purpose. Alternatively, IB has an app and OP could easily have obtained his BP at T-24 (prior to MAD departure).

Second the LHR lounge issue is a non-issue. As a non-elite Y passenger, he is not entitled to lounge access under OW rules. This is made very clear and OP simply failed to check the clearly posted rules in advance. As an AC member, OP is entitled to AC access those, not shockingly, are located near AA not other carrier, departure areas. Just as the AC is a paid for lounge system, there are lounges which OP may pay to access at T6.

Here is a link to the OW lounge access rules:

https://www.oneworld.com/ffp/lounge-access