Look at Kingspan K106 cavity board. It’s a full fill 100mm product, which compresses down to 90mm once in place and leaves a 10mm residual gap. It’s designed for 100mm cavities and allows you to hit the newer u values.
Have just come across this thread on Google while trying to find an answer to this myself.
A couple of things I’ve found so far.
The actual law page for permitted development states:
“(2) Unless the context otherwise requires, any reference in this Order to the height of a building or of...