As other have said, this is all pretty much normal.
Now we are not allowed into the building at all (staff guarding the door). - normal. Many schools do allow parents in to drop off in reception, at least for the first term, but many don't. As a governor of a primary school, the idea of 30+ parents (and others - GPs, random relatives..) wandering the halls of a morning unchecked always gives me the heebie jeebies! I can see it's nice for the parents but the problem is that it is an easily abused privilege.
two parent/teacher meetings per year, stopwatch put on as we walk in the door for 10 minutes. - normal. 30 children in a class - that's 5 hours of parents evening, assuming everyone sticks to their 10 mins.
And one 1/2 hour per year where we can come in to see the children's workbooks (30 children + parents in the classroom at once so can't talk to teacher, just a quick flick through). - normal. Actually, my primary does this once a term, and gives about 2 hours to it so it's a bit more flexible. They also have the books out at parents evening so you can come a bit early and look at them. Maybe suggest this?
The only information we have about what they are doing is the brief summary of the year on the curriculum page, but not detailed or specific and not scheduled. - do you not get any newsletters? Our primary sends this weekly with a brief summary of what each year are doing.
NO feedback on our own child, except 2x10 minutes/yr. - do you not get a written school report at all?
It is made clear that we are never allowed to speak to the teacher, at drop off or pick up. We are told instead to write a message/question to the teacher and send with child. - normal. Whilst many parents would stick to the only talking when necessary/quick mention that little freddie is a bit off today, many more don't and it becomes unmanageable. Often it's better to have stuff in writing too.
I have done this a handful of times and more often than not it is ignored. - that's not great though, of course it isn't.
If you feel that you would like more engagement, a good place to start maybe to suggest perhaps being able to look at books on a termly basis?