Just flew SQ from BKK-SIN-LHR in business class - and had a separate BA ticket from LHR-NCL - and they were able to check my bags in from Bangkok all the way to Newcastle, despite the fact it was two separate tickets. They were even able to issue a boarding pass for LHR-NCL. Made life much easier when landing in LHR as I could get the airside bus from T2 to T5.