I have a roof.
I suspect others may too, so am asking advice.

It's on top of an attic, and has tiles on top of a felt paper layer,
attatched to rotting bits of wood.

The underlaying planks (I'm sure there is a technical term) are sound,
but won't be for that much longer.

Hence, I plan at some time in the not-too distant future to remove all the
tiles (section by section) the old bits of wood which the
tiles are hung on, replace the tar-paper, with nice thick tar-paper,
and then attatch more wood, and replace the tiles.

The only dodgy bit of the existing roof is the flashing at the
edges, which is crap, and that at the bottom of the roof, the tarpaper
actually goes up a bit and forms a valley against the lowest plank at
the edge of the attic, as the brickwork has lots of debris on top of it.
This has caused that plank to partially rot, so this will need replaced
along the length.

At that time, I'm planning to add some exterior grade ply along the edge,
to carry the tar-paper to the edge, in a nice smooth manner.

However, I was wondering about adding insulation at that time.
What would be involved in adding 100mm of polystyrene sheets to under the
tiles?
Obviously, the flashing needs redone (but needs doing anyway), and tiles
at the ridge need raised, and the gable-end needs 50-100mm added on (the
attic beams don't quite come high enough to make the same angle as the
gable end).

I was thinking of removing all the tiles, tar-paper, and bits of wood
that are existing, and then tiling 100mm*8'*4' sheets of polystyrene
all over the roof up to the ridge where it's mitred, using
expanding polyurethane glue.
Then, 4mm exterior grade plywood over the whole lot, followed by gluing
battens on to hold the tiles, then replacing.

Access is awkward, as it's a bit high up, being a roof, but possible
safely.
I'm not in a conservation area, have no near neighbours who tiles will fall
on, and the roof is not shared.

Is raising the ridge-line 10cm likely to need planning consent?
The gable-ends are already 15cm higher than the ridge.

It's a fairly repetitive job, that involves working on a roof,
exposed to the weather, and vulnerable to rain, but I don't see
anything that I fundamentally don't know how to do.
Thoughts?