I recently flew NCL-LHR-CPH/ARN-LHR-MME (timings meant that I could not do the domestic leg from the same airport).

Coming home I was unable to check in online but did it at the desk at ARN. However, the agent was only able to check me in as far as LHR despite the trip being on one ticket and the LHR-MME leg being on the BA codeshare. I arrived at LHR, went to the T5 transfer desk and was told "no we can't help you, this is a LM operated flight, you need to get your boarding pass from T2", I took the airside transfer bus to T2, went through the bag scan and approached the UK border and then the boarding pass check. I was obviously unable to provide a boarding pass either person but both (after checking my passport and asking a few questions about where I was connecting on to), let me into the departure area. No Loganair desks there but when the gate opened I was able to get my BP from the gate agent.

Reflecting that:
a) if I had checked luggage then I would have had a much more complicated journey
b) should I really have been let through into the T2 departure area without a BP?
c) would I have failed conformance if I had not collected my boarding pass (effectively checked in) as soon as the gate opened (and potentially got booted off the flight)?
d) why does the IT just not work (after all BA & LM have a relationship going back years)?