You can only boycott the census if you can afford the thousand pound fine. But the critics of its acquiescence to gender self-identification are essentially correct.
For example, how are we to plan the National Health Service? Some people are going to need cervical smears, while other people are going to need prostate exams. We need to have at least some idea as to how many there were to be of each, and as to who they were to be. Therefore, the question must be biological sex.
People who are making facetious points about Jedi, you can change religion. You can start your own religion. You can have no religion. But you cannot have no biological sex. You cannot start your own sex. And you cannot change sex.
By the time of the 2031 census, then at least one of two things will have come to pass. Either all paperwork will ask whether your chromosomes were XX or XY, while the likes of changing room doors will feature no words, but little signs vaguely reminiscent of penises and vaginas. Or sex will have been removed from the Equality Act on the grounds that "no one" any longer acknowledged the concept. Or both.