WanMil, It's strange. I don't get that error message
Area too small to split at http://www.openstreetmap.org/?mlat=51.87630&mlon=1.24341&zoom=17 (reduce the density of points, length of lines, etc.)
when I run mkgmap-r1955 on the filtered tile that I uploaded (3MB). I do get exactly that error message, three times, when I run mkgmap-r1955 on the complete tile (16MB). That's why I did not suspect that the subdivision splitting problem might be the cause of the missing land polygon. (It is missing in both cases.) Because of this discrepancy, I looked further at tile 63240175, which I described before. I produced a filtered version of this tile containing just the coastline elements. I ran mkgmap on it and got the error message five times, the same as with the complete tile.