Many said by others but:
Benedict, Bertie, Crispin, Crispian, Casper, Darius, Dominic, Giles, Guy, Isambard, Jasper, Jeremy, Jonty, Maximilian, Miles, Monty, Raphael, Rafe, Rupert, Rollo, Sebastian, Timothy, Toby, Warwick. Ones like Charles or Samuel if not shortened.
Arabella, Araminta, Beatrice, Belinda, Camilla, Clarissa, Claudette, Claudine, Clemency, Clementine, Coco, Henrietta, India, Veronica. Also for girls, often the posher version ending in -a instead of -e, e.g. Julia instead of Julie, Emilia instead of Emily, Lucinda not Lucy, Sophia instead of Sophie, Isabella instead of Isobel, Helena instead of Helen. None of those especially posh though, and lots of names like Olivia once posh now fairly common.
Basically many names you might find belonging to a debutante, 'IT-girl'/socialite or in the pages of Country Life or The Lady magazine! Hyphenated too, e.g. Chloe-Jasmine, though that sort of thing has become more chavvy in recent years.