If you are using qgis3 you should use processing.runAndLoadResults()
something = processing.runAndLoadResults("qgis:advancedpythonfieldcalculator",
'FORMULA':'value = $geom.area()','OUTPUT':'...
Technically, OSGB36 is the geographic coordinate reference system that is used as the basis for British National Grid, EPSG::27700. British National Grid uses transverse Mercator as its map projection algorithm. TM is conformal so no, it doesn't maintain areas. It's designed to maintain local angles (shapes).
On the other hand, depending on how accurate the ...
One approach is to convert your polygons to binary raster and calculate the percent building cover within a moving (focal) window for each pixel. Here is how:
Convert the vector polygon building footprints to binary raster
where building = 1, else 0.
Raster > Conversion > Rasterize (Vector
Run a moving window using some areal unit that is ...
Open the attribute table of your polygon layer, select field calculator and check the box "create virtual layer". Than just paste the expression to calculate the area. Define a name for your new field and select the field type.