Yes, this should be your usage on average, but consider fact that you use zero heating in summer months so it has to be offset but much higher usage in other months. It is a reason why they give annual number as following your (mathematically correct calculation) it would be possible to heat this 100 m² with 2.25kW heater which is not true.
Anyway this indicator is not useful for calculating actual costs, because different people like different ambient temps and different years have different weather etc etc Use it to compare two properties, for example your current house needs X kWh/sqm and new one Y kWh/sqm then Xarea_old/Yarea_new ratio would help you estimate how much more energy you need more or less. Of course costs would depend on fuel used.
Not to mention the fact that BER methodology makes lots of assumptions about construction methods (namely, how much insulation and how well installed). As far as I could tell, there was nigh on zero account taken of draughtproofing (bar for chimneys being blocked up). Room for massive difference in cost of heating for two identical (in terms of size and shape) buildings.