A national school-based human papillomavirus (HPV) vaccination programme has been available for 12–13 year old females in the UK since 2008, offering protection against HPV types 16 and 18, which are responsible for the majority of cervical cancer. Little is known about HPV knowledge in girls who have been offered the vaccine. Girls offered the school-based vaccine in the first routine cohort (n = 1033) were recruited from 13 schools in London three years post-vaccination. Participants completed a questionnaire about HPV awareness, knowledge about HPV and the vaccine, and demographic characteristics including vaccine status. About a fifth of the girls reported they were unaware of the HPV infection. Among those who reported being aware of HPV (n = 759) knowledge was relatively low. Approximately half of the participants knew that HPV infection causes cervical cancer, condoms can reduce the risk of transmission and that cervical screening is needed regardless of vaccination status. These results are helpful in benchmarking HPV-related knowledge in vaccinated girls and could be used in the development of appropriate educational messages to accompany the first cervical screening invitation in this cohort in the future.