Andrew wrote:

They do make PB in 3 metre lengths. But you'd probably need to buy
a pallet load.

No, SIG insulation sell the 2.7 and 3.0 boards in singles, you'll
probably get hit for a delivery charge though unless you're buying other
stuff from them (e.g. you might consider buying resilient bars for sound
isolation of the ceiling boards if there's a flat upstairs)