What's the floor U value?
What are the door and window U values and areas?
What allowance has been made for thermal bridging?
What's the ventilation heat loss rate?
The floor area can be considered to have a lower ∆T than the walls and roof, as the ground temperature under a floor, pretty much all year around in the UK, tends to be about 8°C.
Assuming that the floor U value is also 0.2W/m².K, that there is a 1m² area window with a U value of 1.6 W/m².K and a normal pedestrian door with a U value of 1.6 W/m².K, and with a low ventilation rate, for a ∆T of 20°C between inside and out (ground temperature assumed to be 8°C, and internal to ground ∆T of 12°C) then I make the heat loss about 345 W. I've not included an allowance for thermal bridging. Also, if the doors, window and floor U values/areas are not as assumed then the heat loss will be different.