I used the calculator to see what the worst-case [i.e. minimum] separation distance [‘R’ in the Ofcom spreadsheet] would be for my FT857 running 50W on 2m to my Diamond X50 collinear on my chimney, and it’s about 4m.
As the 3D radiation pattern is somewhat doughnut-shaped you would need to be in a ‘cherrypicker’ at the same height as the X50 to get that power density.
I think it would be reasonable to argue – without doing another calculation - that, since the power density drops off as you go above or below the horizontal plane around the colinear [e.g. a line from the X50 to the public road outside my house], the R value will be bigger there than the worst-case 4m.