When do you want the exam to happen?
Teachers know (or should do) where a child is struggling, and the teachers should also know how to differentiate work for that child to support, encourage and then push that child on.
Equally, they should recognise where a bright child is bored, and how to stretch them appropriately. However, a test is not going to tell you this, but teacher assessment will.
I taught a pair of twin boys. Both bright. One an exam animal, one not. The one who was not did worse in his KS2 SATs than his twin, and could not get past that for most of KS3, measuring himself against the label of his SATs level and his brother. I spent 4 years convincing this lad that he was not 'thick' and that he could achieve, and he did. He came to Friday after school GCSE RE lessons and got an A in year 10 for full course, followed by a string of As and As in Year 11. He is now at university and is doing really well. The other twin, the 'bright' one burnt out early and got Bs and Cs and now works for large well known supermarket chain.
The point of this is that SATs label a child at 11, and that is a label they take with them in their heads to secondary. If you purely went by the SATs marks then twin A was on paper brighter than Twin B, but teaching them both, I knew that wasn't the case. I used to tell my year 7s I didn't care what their SATs mark was, it was the level I marked their work at that mattered to me.
When I left junior school at 11 in 1977, I can't recall doing a final exam, but was still put in the A stream for comp.
With teacher assessment it should be checked by other teachers. When we did assessments for RE, we each had to find examplars of a piece that hit the different levels and justify it in a department meeting. When I had a piece for which I couldn't decide a level, then I passed it to my HoD and we discussed it, so there was inbuilt moderation in the system.
Teacher assessment will also give you a better picture because not every child can cope with exams or will have the space/resources/opportunity/parental help to study at home for a series of external tests. By all means do internal tests (I remember Friday morning spelling, definition and tables tests with Mt Sherwin at Juniors very well), but I am not convinced that another layer of external tests will tell you any more than a teacher assessment will at 11.