Dave Liquorice wrote:
A hand saw will do it but generate a lot of bits that'll be great fun
with any static electricity (but probably not as fun as the bits from
expanded polystyrene). Deep score with a stanley knife 'n snap works
for 25 mm not tried that on thicker boards. For thicker borads I'd
try a scallop bladed knife, possibly with some thin wedges to keep
the cut open.

I read that score'n'snap may not give a straight edge on thicker boards. So
you would likely have gaps where the insulation is thinner, and harder to
butt pieces together or have a tight fit against joists.

You can get special insulation hand saws:
https://www.amazon.co.uk/Bahco-PC-22.../dp/B003Y3SJMG

For power tools there's this sort of thing, but it ain't cheap:
https://www.festool.co.uk/products/s...5,2-eb-plus-gb
If you have a big job to do, I wonder if they might be hired?

Theo