Alton Priors church seems to be the exception.
I have Never come across this sort of access to stones inside a church before.
I have seen churches with no visible signs of stones from the outside but visible once in the church (cunning eh?)
and several in the porch (Sussex) so they can be touched on the way in.
I think the Alton Priors mystery lies in Who paid for the the trap doors.
PeteG