I work in a law firm that advises a large number of independent schools on matters of educational law. Although i'm not involved in education law, there is a lot of work going on behind the scenes at the moment on this.
Critically the integrity of the exams has to be rock-solid, and there needs to be a uniform method of invigilation, processing, etc. Without this, the cries of "unfair!" will certainly arise quickly and in their droves. I don't speak for anyone advising the schools, however the professional school of thought (ignore the pun) is that mock exams shouldn't go ahead until further guidance is release by the government. The risk of opening a school up to litigation claims of either malpractice, unfair treatment, cheating, etc. will otherwise besiege independent schools for years to come.
Until the government publishes a rock solid plan, which can be applied to all schools (not just those who can afford / believe in, fancy home invigilation software systems, of differing quality), which also comes with a robust defence mechanism to protect from parental law suits (when Jimmy doesn't get the 4 A*s that his parents thought they'd paid for) then doing mocks is pointless.
Until the plan is clear, simple and one that can be used across the board (perhaps that will be mocks!) then the advise will likely be to hold fire.