You would have to protect against crushing and movement. For movement, I bought a big bunch of cable ties at a thrift shop and tied everything together. That takes care of movement. Padding and support is not so easy - I don't have a Brommie so can't assess it but I would be concerned about the chainring bending and the frame getting kinked. So that's where I would pad it.
Some duct tape around the whole lot to make it into a tight rigid parcel may help as well.
One chap has taken some baggage handler advice on board and put his bike unpadded in a clear plastic bag! The handler said that parcels are uninteresting and just get tossed to the next stage, but the transparent bag showing the bike plainly got a lot of care.