Well i have heard it is quite difficult.
but when i was a primary school.
we had a pianist/teacher.
he would play at assemblys and what ever.
he had never done a grade in his life and he played the organ with out pedals but it still sounded ok

i know for a fact that lessons arn't that hard to get hold of.
but organs are very expensive.
i should think at grade 5 your well past the standerd to beable to play one.
so i really suggest you just go for it and do aswell as you can,
if your lucky they might even give you a full time job at the church for it

hope so.
good luck mate
Davé