Tamsin Egerton

Tamsin Olivia Egerton-Dick (born 26 November 1988) is an English actress known for her roles as Chelsea Parker in the 2007 film St Trinian's, Holly Goodfellow in the 2005 film Keeping Mum and Guinevere in the 2011 television series Camelot, one of three different roles she played in TV series connected with Arthurian legends.

https://en.wikipedia.org/wiki/Tamsin_Egerton

Note: Private is her half sister (shared mother).