Who is the girl singing in churchill karaoke advert?

The model featured in the advertisement is the British actress Lily James. James has appeared in a number of stage, film, and television roles, including her breakout role as Lady Rose MacClare in the ITV historical drama series Downton Abbey (2012-2015).

She has since gone on to star in films such as Cinderella (2015), Baby Driver (2017), and Mamma Mia! Here We Go Again (2018).