For 3d projects, and adding strength to normal projects, I highly recommend the construction board (greyboard) from Funky Pen:
They stock the commonly used 750 micron, and the 1000 micron board in both 12″x12″ and 12″x24″ sized sheets.
Their prices start at around 22p a sheet…
For example at the current date (Jan 2019), 100 sheets of 12″x12″ 750 micron costs £22.55 (that price is inclusive of postage and packaging).
Again, please feel free to ask your fellow ScanNcut users on the various UK Facebook groups to verify the quality and value for money of Funky Pen’s greyboard!