Thanks Lizzy Zoom for making the point and yes my gender does not match my birth certificate , the reason for letting them know is that Gender services are woefully under funded so if they can see the % it may improve.
Yes, there are many trans men and women who have yet to be officially identified and have obtained their true status under the Gender Recognition Act 2004, so the Census should reveal an up to date picture. The government should already know though the number of people that have obtained a Gender Recognition Certificate.
What it will not show is the number of children who already identify as the opposite gender.
It is an extremely interesting very serious field of scientific study and requires far more support than it does to avoid many lives being terminated far too early.