I can't see how cancelling exams and relying on teacher assessment will make things more fair, really. It can't - because it simply isn't fair that some students missed a lot more school than others. Nothing can really change that.
How can teachers possibly give fair grades to a situation which is unfair to start with? They can't. Do they say 'well, if this student had been in school for the whole time, they would have got an 8. But they've only been in for a couple of weeks here and there and haven't learned much of the material, and only score a 5 on the assessments we've done. So I'll give them an 8, because I'm sure that's what they would have got if things had been normal'. Or do they give a 5, which is reflective of the work that they actually can do by this point? One method is unfair to a specific student who didn't get the chance to achieve his best because they had to miss so much. The other method is unfair to those students who do know the work needed to achieve a level 8 and are able to do it when this student can't. And it all makes it difficult for moving on to sixth form, too - unfair for the student if he can't move on to the course he always intended to do, because he only got a 5 because he missed so much. But unfair to the other students and teachers if they have to start teaching GCSE level work instead of A-level, because that's the level some people are at. Or unfair to the student to let them onto the course but then start teaching at the normal A-level point, when they've missed out much of the earlier work. It's just unfair all around, however it's done.
I don't know what else they could do really. But I don't really see that exams would have been much worse than teacher assessments in the fairness stakes. (Though possibly it's easier to cope with possible disruption to the timetables, people isolating during the actual exams, all that side of things, if it is still going on by then - though hopefully not!). But in terms of fairness in terms of how much pupils have missed, well, there's not any good way of overcoming that - they needed to be on top of it much earlier to try to prevent some of the unfairness from happening, by making sure schools were able to deliver high quality online teaching, everyone had devices/broadband, more places available for those who don't, more catch-up programmes over the past months and in the future, safer schools that might have allowed them to stay in-person for longer. etc.