Before spending on the plinth, it's worth confirming where the heat is going. On a cold morning compare the floor surface temperature mid room with the strip along the external walls using an IR thermometer or thermal camera. If the edges are a couple of degrees colder, you have a perimeter bridge and the plinth is worth doing. If the whole floor reads much the same, it's more likely the finish: tiles on a dense slab feel far colder underfoot than a suspended timber floor at the same temperature, and 150mm of concrete is slow to warm.
As your inner leaf sits on the slab, heat can leak from the slab edge through the blockwork and down into the ground beam, so also check that the cavity fill runs down to at least the level of the underslab PIR rather than stopping at DPC.
SimonD is right that it's normal practice. If you go ahead, keep the plinth and its capping below the DPC so nothing bridges it, leave any weep holes clear, keep finished ground level 150mm below DPC, and bond the XPS rather than fixing through it below ground.
