With Youth:

https://www.withyouth.org/living-with-anxiety/

Services for young people:

https://www.servicesforyoungpeople.org/support-for-young-people/information-advice-and-support/

Childline:

https://www.childline.org.uk/info-advice/school-college-and-work/school-college/exam-stress/

Mind:

https://www.mind.org.uk/information-support/your-stories/14-ways-to-beat-exam-stress/