Books the top set my DD was in was doing this kind of algebra by the end of Yr5. This is in a really ordinary state school, BTW - we are not in a grammar school area so no 11+ issues. They did however happen to be a very strong year cohort.
She only had additional lessons in English, a grand total of 6 half hour sessions in the run-up to the SATs (3 per week during lunch hour for the children selected to do the L^ paper). I'd count that as a handful, wouldn't you?
I never said that a bunch of kids passed L6 though - my DD was one of 3 who passed in English and one of 2 in maths. As I said, her school is an ordinary state school in a non-grammar area. Their intake is very mixed (think a lot of socio-economic deprivation and parents who don't give a stuff) but they support the children very well. DD said she found the papers tough, but enjoyed doing them.
I have done nothing additional with her at home. Nothing. At. All. I make sure she does her homework, I make sure that she understands what she is doing, but I do not purchase additional practise materials for her, nor do I make her do extra work. I don't understand why you doubt this - she is very bright, and has had very good teachers. I'd have been happy for her to not do the L6 paper, but she was asked and she wanted the challenge. I realise that she has been very lucky, both in the brain she was born with, in the teachers she has had and in having a stable and supportive home environment.
I have no doubt that practice at home will make more kids pass L5. However, I question the necessity - I know that DD1 will be sitting CAT tests very early in the autumn term, because the school she is going to knows that a lot of their new intake will have been drilled into scoring high on SATs.
And no, I am not going to do any CAT practice with her either. I'd rather know what her real standard of education and knowledge is.
I'm not a teacher, but I am a qualified trainer with knowledge of how people learn, which may have helped my DD in terms of the support she's had at home with her homework - this purely in terms of me being able to encourage her to edit her own work, identify and correct error and study effectively. And yes, I admit that has probably given her an advantage, but it isn't the same as getting in worksheets and doing hours of extra work.