Apologies for not reading the full thread (80 replies in 3 hours is impressive!) but just had to say how much I LOVE your list. I wouldn't pick Caroline or Francis personally but I think all the others are fabulous and go brilliantly with Penelope. My favourites are Jemima and Jessie with Robin a very close second.
1970s slightly upper crust makes me think:
Annabelle, Alexandra, Belinda, Bryony, Cassandra, Clarissa, Calliope, Calista, Charlotte, Dominique, Eloise, Emmeline, Felicity, Francesca, Georgina, Gabriella, Henrietta, Harmony, Juliette, Joanna, Jacqueline, Jocelyn, Katherine, Liandra, Larissa, Miranda, Millicent, Melisande, Melissa, Natalie, Natasha, Philippa, Phoebe, Rosalie, Rosemary, Rosanna, Rowena, Selena, Saskia, Sylvia, Tamara, Vanessa, Veronica, Victoria, Virginia
Boys much harder - I suspect most of them were the solid, traditional boys names you still get. I think you've nailed it with the ones you have already but maybe:
Alistair, Benedict, Christian, Elliot, Edmund, Fergus, Finnian, Hugo, Hugh, Jasper, Jonathan, Justin, Jude, Jeremy, Mungo, Monty, Nicholas, Patrick, Quentin, Solomon, Silas, Timothy, Vincent, Victor, William