Whatever the top grades were. I believe, way back when I did them, that the highest grade was 5. I recall being told I answered only one question wrong in the maths paper which was annoying as it was so simple. I was an insufferable child and would have far preferred to be able to tell people I scored 100% I was that child who would tell teachers my IQ when they were telling me off
I went to a private school that didn't do SATS. On our end of year equivalent tests I apparently was working on a low level 6 for English, a mid level 6 for maths and a high level 5 for science.