Just like today, dictionaries and other reference works in Georgian Britian often provided lists of Christian names. The remit of these lists varied: some included only the most common names; whereas others included a wide range of uncommon names that were known in Britain through history, mythology, etc.
I found eight distinct lists in books published throughout the whole of the eighteenth century (assuming that fashions in names did not change so very quickly, especially with the likelihood of a child being given the same forename as their father or grandfather). Names that were included in all eight books can be thought to be truly well known and widely used.
Found in all eight books:
Agnes
Barbara [written Barb^a] (diminutive Bab)
Dorothy [Doro] (Dolly, Dol)
Elizabeth [Eliz] (Bess, Bet, Betsy, Betty) {see also Eliza listed in its own right}
Grace [Gra]
Hannah [Han]
Isabel (Ib, or Nib)
Jane (Jenny, Jinny)
Katherine (Kath, Kate)
Margaret [Mar^t] (Peggy, Peg)
Martha [Mar] (Patty)
Mary [Ma] (Molly, Polly, Poll, Moll)
Rachel
Sarah [Sar] (Sally, Sal)
Sophia (Sophy)Seven out of eight books:
Abigail (diminutive Nab) Agatha Alice (Assy) Amy Anne [written Ann] (Nancy, Nanny, Nan) Beatrice Briget (Biddy) Cassandra (Cass) Constance (Conny) Deborah (Deb) Diana (Dy) Dorcas Edith Elianor [Elean] (Nel, Nelly) Eliza Emma Esther (Hetty) Eve Faith Flora Florence Frances [Fras] (Fanny) Joan (Jug) Joyce Judith Lettice (Let) Lucretia Lucy Mabel Magdalen (Maudlin) Mercy Patience Penellope [Pen] (Pen) Phillipa Priscilla (Pris) Prudence (Pru) Rebecca [Reb] (Beck) Sabina Susanna (Suky) Thomasin Ursula [Urs] Winifrid [Win] (Win)
Six out of seven:
Alethea/Alathea, Arabella (diminutive Bell), Avice, Bertha/Birtha, Damaris, Eunice, Gertrude, Gillian/Gilian, Hagar, Hester (Hetty), Jaquet, Jennet, Lydia [written Lyd], Maud/Mathildis, Melicent, Mildred/Meldred (Mil), Phillis/Phyllis (Phil), Phœbe, Rhode, Rosamund, Rose, Ruth, Salome, Sibyll, Tabitha [Tab], Temperance.