As many others have said, the reality is that reception and even Y1 in the UK are similar to kindergarten/preschool etc. in other countries. Not that much difference.
I think the difference bx the UK and countries where the mandatory age starts a bit later (which is most countries, to be honest) is that the inclusion of the 4-5yo in the schooling system results in a more gradual/incremental process.
UK kids officially "start school" at 4 and do a bit of academics from the start, but it is mostly play at that age, and then the academics/play ratio decreases veeeerrry gradually through the primary years. I am generalizing a bit, of course.
Even in the later years of primary school, British primary classrooms are still quite "kindergarten" looking to my eyes--students sitting round grouped tables or on the carpet, teachers walking around from group to group, lots of chatter, not many textbooks etc.
In Japan, where I live, by contrast, school doesn't start till 6 but when they start, they really do START SCHOOL and it is a very clear break from the early-years stuff. They have rows of two-seater desks facing the front, textbooks for every subject, whole-class instruction most of the time and so on (they do put tables in group for things like actual group work in Science class etc. etc., but it is not the default arrangement). To a lesser extent that tends to be true of most countries where kids "start later," I think. In France and Germany and even Finland, once kids start actually start "proper school" at 6 or even 7, they usually sit in rows and make extensive use of textbooks and whole-class teaching.
I suppose which way one prefers will depend on what one likes to see in the classroom.