I am also a platinum member, I have a sheltered balcony cabin on deck 4 (QM2) and I have been assigned 3PM for a final boarding time at 4PM. There's no mention of Platinum, or priority boarding, on my boarding pass, but "loyalty tier: red". I had to reach out to CWC because mycunard.com initially showed "no loyalty tier" (whereas the correct info was on cunard.com). They changed it but the boarding passes may have been issued before that. I haven't had a chance to cruise with Cunard these last years, but I do remember that the last time I did, being a Platinum member allowed to board the ship pretty early. Which was nice.