None of those names mentioned have the "poshest" of English accents - upper class RP, aka "Fraffly". The best way to get an idea of how that sounds is to think of them say "Weh sew fraffly gled yorkered calm" for "we're so frightfully glad you could come". Penelope Keith is probably the poshest accent, but to a certain extent she has something of a "stage" accent - not a real posh accent, but one that people sort of thinks exist. Joanna Lumley has an unusual posh accent, as its the "Brits Overseas" sort - you can hear that Vivien Leigh, another "Anglo-India" had much the same accent, as does Paddy Ashdown (though the latter has been moderated over the years). A dying accent, alas. Rebecca Hall is probably the "poshest" on the list, but she drops her "t's" and affects something of a mockney accent and for that should be shot.
Problem is I can't think of an actress with a genuine fraffly accent; the sort of people who do have such an accent generally don't go onto the stage!