Can anyone tell me why I can't post anything on 11 plus Rehab. Is it because I am not a member (I can post everywhere else)
Yes. The PhpBB software used to run these forums allows fine control over what you can do and where you can do it - depending how you've authenticated yourself. Have a look at the bottom right hand side of any forum page.
Perhaps it's an accident how 11 PLUS REHAB has been set up or maybe they thought a forum where topics get more than a thousand replies wouldn't be too easy to read with multiple 'guest' posts.
You could check with forumadmin, though I'd just register if I were you - it makes things easier to read and the ability to edit your own posts is useful sometimes, I find.