TL:DR
Ireland is quite a high-performing country in education. It outscores the UK in most international tests (yet gets surprisingly little attention when education is discussed).
I too don't get the resistance to textbooks in England, Scotland and Wales.
My experience of sending my child to school in a (also high-performing) system that uses textbooks is:
It is easy for parents to see what children are doing at school, what they have covered previously and what is coming up next. So you can help effectively at home (it is especially helpful if you speak another language at home and your child sometimes misses things in the classroom). It also makes it easier to revise stuff learned previously or introduce content that will come up later in the course, so that children can practice content several times (very very important for retention--"spaced practice").
It (provided the textbooks are decent quality) ensures that all schools cover similar contents and means that stuff can be taught systematically, building on what has been learned before. So you do not wind up with a teacher trying to teach a "topic" on the Caribbean, and then discovering that his/her lesson is not working because half the kids have never been taught maps/compass points or don't know the names of the continents etc. This helps children learn more efficiently and makes things easier for teachers. It also means that children who switch schools don't end up (say) "doing" the Fire of London twice in a row and never covering Ancient Mesopotamia etc.
If you look at England (leaving aside Scotland and Wales for the moment), English primary schools actually do well in international assessment--look at TIMSS and PIRLS. But then in PISA we sink down to mediocre. Something seems to be going wrong in Key Stage 3 in English education. I think part of the issue is the lack of a good foundation in the foundation subjects at primary, and the fact that kids come in from primary schools all having studied a variety of different bits and pieces which keeps changing year by year. It is going to be very very hard for KS3 teachers to plan a good curriculum or teach effective lessons which the students can understand and feel interested in, in such circumstances. Having content which is unified across the curriculum would surely help this. There could be a proper KS3 curriculum that leads on from a unified primary curriculum in a systematic way.
Finally, use of textbooks increases equity in the curriculum IMO. It worries me when I hear vague comments about a school choosing a curriculum that is "relevant to our particular students' interests etc. etc."--in practice, I think this tends to mean that schools in lower socio-economic districts wind up teaching a less challenging and academic curriculum, which ultimately is not in the kids' favor.
It greatly reduces workload for teachers (not in Japan, but that is because the Japanese system wastes teachers' time with other stuffthey spend little time on lesson planning or making stuff from scratch). One consequence is that teachers have more time to spend working one-on-one with struggling pupils or even just if a child has not understood one particular thing or lesson. My daughter had some issues when she started due to language barriers and being the youngest in her class, and I deeply appreciated the way her teacher was able to sit with her and spend time going through stuff she struggled with. She really benefited from her teacher having the chance to do this. I am sure UK teachers would love to do more of this toobut they probably don't have the time. In countries that don't use textbooks, it seems to be common to employ lots of TAs who tend to work with the struggling students. However, the quality of TAs does vary a lot and some TAs are less qualified than actual teachers. You really want the MOST qualified people to be working with weaker students, IMO.
Expense-wise---it does cost money, but probably saves in the long-run. I think the trick is to make sure that textbooks are reasonably concise, not massive doorstops. A trained teacher can use a textbooks as a starting point and expand its contents as he or she wishes.
Use of textbooks does make for heavy school bags though!