My DD9yrs had very similar scores when tested at 7yrs5mth:
Similarities 16
Vocab 13
Compreh 14
Block Design 14
Picture Concepts 11
matrix reas 12
Digit Span 8
Letter Number Seq 6
Coding 15
Symbol Search 9
Her comprehension and Letter Number Sequencing scores were appreciably lower than your DS, and her coding was appreciably higher. The rest are VERY similar.
She has sensory processing issues, retained reflexes and has just this month been diagnosed with Auditory processing disorder. We are sure she has dyslexia and ADHD and are working on getting those formalised at the moment. Her first school kept assuring us that she was so bright that it was just a developmental/maturity thing and it would all click when she was ready and she would catch on soon. We changed school at the beginning of yr2 and they took the opposite approach of "She's so bright there has to be something wrong", so the testing started. She's had very intensive support at school and home and has now caught up with not just the national average for age, but her school's average (which is well above the national average). She's turned back into the happy and confident 4yr old we knew before school started, it was a rough 4.5yrs in between.
Learning to read was absolute torture for my DD. She used to squirm around next to me on the couch trying to inch away up the back of the couch like she was being physically tortured. We could look at "The cat sat on the mat" and she would not be able to remember the first letter of any three letter word by the time we got to the third (naming each one was agony in and of itself). When we moved from "cat" to "sat" she would not recognise the "a" or the "t" or the "at" sound. If you turned the page and saw "The cat sat on the hat" she would not recognise she had seen any of those letters or words only seconds before. She was verbally precocious but simply could not comprehend any sort of mapping from the language she spoke to symbols on a page.
Interestingly I looked back and realised that we had had the same problem with the symbols for numbers. At some point around 2.5-3.5 yrs old I had realised that her math concepts were way in advance of her peers but they all knew the symbols for numbers and she did not. She had one to one correspondence and basic addition and subtraction, they had counting in order and symbols... It took me MONTHS of looking at letter boxes as we walked every day to teach her the symbols 1-9.
Despite all expert advice to the contrary (and what I can see works really well with DD2) having pictures related to the text did NOT help my DD learn to read. Partly because she could decode a picture with the best of them and fake reading, partly because if there was ANYTHING other than the words to look at that was what she would remember. She would not map the picture of the cat to the word "cat" she would just remember the picture. She did not start making any progress at all until we started doing the alphabet on starfall.com and flashcards of high frequency words and word rules (for example I made cards with "can" on one side and "cane" on the other, "tape" and "tape", "pin" and "pine").
She did preschool and two years of formal school at the first school that thought it would all "just click" and in that time she had reached fairly reliable recognition of most of the alphabet (though not both names and sounds for every letter) and that was all. And that was only after starfall and flashcards. With amazing special ed support at her second school she has caught up but it was hard slog for everyone. She's now starting to show how fast she can learn concepts and how deeply she can understand them, but she still misses lots in the classroom too. We often have no idea whether her understanding of what is required of her is correct or not.
I don't know how access to services works in the UK (we are in Australia) but I have found seeing a paediatric sensory OT and an audiologist invaluable. We currently have appointments booked with an behavioural optometrist and a developmental paed. I am going to PM you some notes from a recent conference I went to that really changed my understanding of inattentive ADHD (previously called ADD in Australia) and left me convinced this applied in my DDs case.
Note: my mother in law reports that my DH learning to read looked exactly the same as my DD, specifically looking like it physically pained him, and was an equally slow and agonizing process. He's now an extremely competent technical reader and can scan for the information he needs with the best of them, but he reads fiction at about half to a quarter of the speed that I do. And when reading aloud he mispronounces words he would never pronounce that way in conversation ("said" for example). My DH and I were both late readers and generally late bloomers academically.
Oh and one more slightly "out there" idea. You may have heard the Steiner theory that children are not ready to read until they start getting adult teeth? Well grasping at straws I asked our special ed co-ordinator about this and she first said "No, I know nothing about Steiner theories and don't approve of their delaying reading" when I clarified about the teeth she said "Oh of course - whenever kids come to me I ask to see their teeth and for the harder cases I don't expect progress until they start getting adult teeth." In her experience there were plenty of kids that learned to read with ease regardless of the teeth situation, but for the kids for whom it was not easy there was some developmental issue that was timed with teeth and their reading did improve around the same time they started getting adult teeth. And this was a factor for my DD who is a long, slow, late teether.
