The exam boards won't just go off mocks which are usually a few grades lower than the real thing, they'll go (if they do this at all) of a combination of mocks, in class tests, other pieces of work- my dd for GCSE has tests or presentations in class several times a term so there should be a lot of material they can draw on.
The cohort will then be standardized to whatever the last two years distribution was- so if you got a mock C, it may be standardized two grades higher into an A.
That's just one way of doing it.
It's not 'fair' as all assessments favour one group or another- those who are better at exams, those better at coursework, those with stable homelives, those in better schools with stable teaching staff and so on. But in general, if someone did two years of solid work, they will get a solid result from this type of system.
That said, I don't know what the government will do, but usually there is a way of standardizing or changing marking boundaries to get the desired proportion of results. If your little Jonnny did nothing for two years but was hoping to ace a 9, it won't work out so well for them but that might be a good life lesson at this stage.
Those crying over SATS? Really? We should be glad a whole year get to skip that madness, they might actually have better mental health than those pressurised age 10 to perform.