I am in one of the areas that took part in the test census so I completed my last night.
And I am ashamed to say I forgot to screencapture any of it! 
But I did concentrate on the question about sex. It was very straight forward ie male or female and after that it just said there would be a question about gender later on.
I clecked on all links on that page and nothing came up about assigned at birth or sex you identity with so I suspect most people will just respond as the normally do, ie without thinking.
The later question about gender states it is optional and suspect many will just pass it by. It did have an open definition answer so I put in that nobody was a gender and that gender is a social construct.
But at least all those disenfranchised Jedi from the last census could use it.
But ... I am not saying phew nothing to worry about as:
i) as a test version it maybe didn't include everything
ii) I am notrious for not seeing obvious links, footnotes on web pages so may have missed something
They do give an email address if you have a comment or feedback, and haven't yet contacted them.
Just wondered if anyone else has done the text census and can let me know whether I did miss something. Dont want to email in comments that only show I wasn't pay attention!
On another issue about the purpose of the census being to help with future planning it seems totally useless and for some reason focused on education qualifications. By that I mean it is quite a long set of questions and may be useful in relation to a younger age group but seems totally irrelevant to middle to late age. I cant see what they are hoping to achieve.The only thing I can think it that the questions on employment where more than just job title but an explanation of the type of work. Is this to show that further education is pointless because it turns out those in top jobs / loadsamoney left school at 16?
Feel very dillusions with the whole process and the can of worms that trans activism is. Of all the pressure groups in the UK does the Government really feel that the most important one is the trans community so they get to have their very own special question?