I wonder whether there is less impetus to have memorials- especially public memorials- when the deceased in question is a true, major league celebrity like Angela Lansbury? Her life and career were so honoured in all the media and presumably there was a service for her nearest ands dearest.
It's the mid-level figures who aren't necessarily eulogised in the press that memorials seem really best suited for. I think of wonderful actors like Elizabeth Wilson or a choreographer like Inna White, whose names are unknown to most people but had a coterie of devoted fans and admirers who were longing to gather to celebrate them once last time.
But if there IS a public memorial for Dame Angela, I would love to attend!