I think there's been quite a divergence this year between pupils depending on home situation. Some primary kids have good internet, parents who can find the time to sit with them to watch any video input, help them understand it, encourage them with set tasks, and maybe even build on that. Others have little or no access to a computer, and parents who can't sit with them because they're working or looking after other siblings, and who might not be able to explain the work when they're stuck anyway.
There might be a lot of sense in allowing some pupils to repeat the year, but the logistics might be tricky, particularly in smaller schools. If you have 30 kids in each year, the chances that the number wanting to repeat each year matches the number from the class below who want to repeat that year are slim. Mixed-year classes might be necessary. And there would need to be some extra capacity, both in terms of staffing and classroom space.
There's also the problem of whether there's a bit of stigma attached to repeating - not only for the children ("but I want to stay with my friends"), but also for parents who may see it as labelling them as having failed with the home-schooling. I'd like to think that repeating could be considered as an option if the logistics work, but I think plenty won't want it.
Fortunately teachers are usually very good at differentiating for pupils with different starting points, and I think they will all be acutely aware that there may be rather bigger differences than usual, and also that some children may catch up quicker than others. In recent years there has been an increased emphasis on identifying children who are falling behind expectations, and putting in place intervention to help them catch up, so that's also going to be more important than ever.