If a stove is placed on a combustible floor then the hearth must be at least 250mm thick (or 125mm thick with a 50mm air gap underneath). This hearth thickness can be reduced to 12mm if the stove does not raise the hearth temperature to 100 degrees centigrade.
Many stove manufacturers have tested their stoves and should be able to state whether their stove does raises the hearth temperature to over 100ºC or not. Contemporary stoves, with wood stores underneath, are very unlikely to heat the hearth to more than 100ºC.

You should bear in mind that if you are using a 6mm glass floor plate or a 2mm steel plate on a combustible floor that you should place another non-combustible layer under the floor plate to bring the total up to the 12mm required by regs.