We had a very comprehensive set of testing done for DD when she was 7 (done via her state primary in London) and then repeated in Y6.
She's quite severely dyslexic (and also ADHD - although that wasn't looked for at the time). Didn't learn to read until Y3 and spelling and punctuation are still horrendous. DD also has hypermobile fingers so writing is very painful.
We tried everything - Nessy, Toe-by-Toe, sheets of common words, overlays, weird shaped pencils, dyslexia friendly reading schemes... the lot. And none of it made any visible difference although they were possibly helpful along the way.
The only thing that has been a huge and life-changing factor was teaching her to touch type in Y6 and moving to a laptop for everything with grammarly and spell check installed.
She started secondary with this and has used it ever since (we have an iPad with keyboard and e-Pen rather than a laptop). The EP reports were brilliant as they outlined where the issues were - v spiky profile with very high VR and NVR scores and incredibly low working memory. The report also stated that DD should be sat at the front of classes and use a keyboard at all times - so has been vv useful for waving at schools and for things like extra time and a laptop for exams.
We never went down the route of an EHCP - I couldn't see what help DD would get from one given that schools all said they would put things in place to help and yet we never got anything.
For DD it was very helpful - she knew that she wasn't stupid, just has an SpLd. She would definitely have failed SATs, has never achieved in-line with her potential, but secondary seem to get her and she's predicted top grades for GCSE (not sure she'll get them, but at least nobody is under-estimating her just because she can't spell and still finds reading very tricky).
Audio books and films are also a godsend if they won't/can't read.