https://find.redbridge.gov.uk/kb5/redbr ... o8CR1SQQKwThis link may assist with a general discussion and local advice.I would suggest discussing your concerns with the english teacher , the headteacher of the school and the LEA.It would also assist you to get hold of the schools SEN policy.You have highlighted one of the difficulties in that children brighter than average since they appear to be coping are not having their disabilities diagnosed.I wouldn't necessarily assume your child has dyslexia until you have a definitive diagnosis.Clearly she is showing signs of dyslexia but it may be another condition or conditions combined.
The local authority are likely to have both experienced psychologists and experienced teachers who can do a report in assessing learners with dyslexia/specific learning difficulties.A proper assessment can take the best part of a morning and can take place in school.The report should not assess general IQ,but should address and could include:
Literacy skills and attainment,
Written skills,
The wide range Intelligence test,
Comprehensive Test of Phonological Processing,
Working memory test battery for children and British Picture Vocabulary.
What in particular the assessor and you will need to consider any scores which fall at 85 or under.
Through the local authority it is free for the assessments.If this route is not possible you will have to consider other routes.