"Gosh Mumsnet isn't for the faint hearted is it" 
Have you not been around long then OP?
This is nothing NOTHING, compared to the flamings i have had on here! But don't go! Mumsnet is very straight talking so the advice you get is generally good and its supportive, however you don't always hear what you want to hear.
I think a few things might have put PPs backs up. The comments about SEN and pupil premium made me feel a bit
As a parent, you should have no idea about whether a child has SEN (unless its blatantly obvious or you know them) and even less so pupil premium.
When my DD's school took DD, i was told she was selected on the basis that she has confidence issues (she is dyslexic which causes this), i know another child that went whose parents were separating and having difficulties because of this. So not always down to SEN or pupil premium (very small % of pupil premium at DDs school). They only took four children from each year as it was a pilot, They relied on parents to help transport the kids to the forest school and my DP did do a couple of the runs. I am hoping that they are continuing with it as it was such a positive experience for my DD but it really does come down to money.
If you can get the PTA behind you to raise money for this i would imagine the school would snap your hands off (PM me if you want advice about fund-raising, i am a demon fund raiser and can give you advice on where to apply for funding if you are interested).
I think it is a bit rubbish that the whole year group don't go, but also think its nice that they do it in small groups as i think that is one of the reason's why it is so good iyswim.