A Diagnostic Assessment is only necessary if you really want to find out if you have a Specific Learning Difficulty such as Dyslexia or Dyspraxia
The best way to find out if an assessment is necessary is to make an appointment to meet with a Disability Adviser to discuss your difficulties and they can advise on the best course of action.
If you want to apply for a Disabled Students' Allowance for support with your Specific Learning Difficulty, you will need to provide a copy of a full diagnostic assessment.