Have you talked to your parents?
I don't know U.K. policies, but if your parents understood the situation, would your parents be willing, or have the option to home school your brother, or send him to a private school?
I was home schooled until third grade and my kids until middle school, so we were all about five grades ahead of our public school peers. Both my kids were reading encyclopedias for fun before they were five.
By the time my kids entered public middle school, when I had to work full time due to divorce, they tested at a college level, with scores high enough at the age of ten to enter the teacher's program.
Public school bored them, so they took books to school to read while the teacher droned on and on, wasting their time.
Teachers were afraid of confronting them because if they tried to call on my kids to give answers-trying to humiliate them for not "paying attention in class," not only did my kids know the answers, they knew far more than the teachers.