Yes the rule here is
- Same PNR and ALSO oneworld, separate tickets = through checked luggage
- Same PNR, not oneworld, BA's through checking is limited to the relevant ticket end point. So
125-nnn1 NCL-LHR-BOS-CLT
279-nnn7 CLT-SFO
You have to recheck at CLT (as well as BOS for other reasons).
- Different PNRs: no through checking, even BA to BA.
The OP will be aware that UK/anywhere to USA involves a customs related recheck process at the port of entry in the USA regardless, unless pre-clearance or ITI apples.