I have been a teacher in Primary schools around London for more than a decade and this was always a contentious issue. Some children love an extra, independent challenge; some parents demand it; some children simply cannot cope. What does the LAW say about homework?
Frustratingly little. There used to be a stipulation in the Ofsted School Inspection Handbook but even that was vague. It stated that, for schools to be deemed Outstanding or Good, they had to 'set challenging homework, in line with the school’s policy'; however, this assumed that the school policy included a section on homework.
The Department for Education said definitely in 2018 that there is no legal obligation for schools to set homework, and went on to say (when pestered) that any sanctions around not completing homework should be clearly laid out in the school's behaviour policy... which is something that must be on the school's website by law.
So I looked at a few behaviour policies of schools from across the country (Cornwall, London, York, Manchester, Cheltenham). There was not a mention of homework anywhere. Many of the schools had a clause that stated things along the lines of school rules must be followed or that children should follow instructions given by all school adults, and I guess 'do your homework' falls under that remit but it's a bit of a stretch.
There are government guidelines saying that homework should 'avoid excessive time requirements'; that any homework set should focus on 'quality rather than quantity'. There was also the assumption that any homework set would have a 'clear purpose'.
The key thing for you as a parent is communication with the school. If you think your child is getting too much homework, either tell the teacher directly or send an email. Don't go straight to the Head, that's not fair. Go to the class teacher first.; they might surprise you. I know that when I had parents tell me that the homework was too much, I asked them to write an official letter to that effect and that their child didn't need to do the homework. I would still set it (I set my homework online way before it was cool!) so they could at least have a go - I didn't want them to feel excluded from anything - but that pressure to complete it was gone.
Another thing you could try is to suggest to your child's school that they switch to project-based homework. A choice of 5-6 activities, each using a variety of skills, all linked to the overall topic of that half term. This was very popular with my classes and it meant that I, as a teacher, did not have endless additional marking to do every week. Win/win!