So just to add, we flew yesterday. We were staying at the Premier Inn the night before so we walked across without the cases & spoke to TUI & yes we could use the twilight check-in for the Cunard flight. We walked back with the cases, passports & flight number & checked in - no hassle & no queues.
The next morning when we spoke to the Cunard Rep at the airport she said that Cunard don't like you doing the night before check-in in case you fall ill overnight.