Madam/Sir Westkate.
What was your expectation of your daughter.You knew she was below average and so it was incumbent on you to make plan A,B and C.
Life is more than good grades.We have many successful people who had low grades but were good in other things.
For now based on her age,I recommend you look for a private school and enroll her there.This will help her grow and learn afew other things eg Social skills,some extra english and swahili,create a network of friends she can relate with in future and broaden her angle of vision.
In the meantime observe what she likes and encourage her to excel in it.
After 4th form since you dont expect her to go to Uni...explore fashion and design(read tailoring),cookery,embroidery,housekeeping,trade(buy cheap at point A and sell at a commission at point B).
Do not give up.You must be strong for your child.
Love is beautiful and so are those who share it.With Love, Marriage is an amazing event in ones life time, the foundation of joy, happiness and success.