Hi OP
Children who don't do well in primary school fall into two groups. There's the kids for who it eventually clicks - and you've had a couple of stories of those on the thread. And then there are the kids where it doesn't, and people don't tend to talk about those.
One of the reasons primary schools are often seen as quite keen to diagnose kids is that they want to find out if there is a reason why the kid isn't "clicking". So a child with dyslexia, is going to need different teaching to that offered in the classroom so that she can progress.
The reason that people are suggesting referral to an educational psychologist on this thread is because of she is in bottom sets at year 7 and has struggled with English and maths all through primary the chances are there is a reason why the teaching she has been getting isn't working for her.
Once you know that reason, then you can work directly on that. And improving that will mean that the teaching she is getting will have an impact.
A boring example - my DS had trouble changing the focus of his eyes quickly. He could see okay, but when he had to copy from the board he was very very slow because he'd look at the board, slowly focus on the next few words, remember them, look down at his book, slowly focus on his writing and write them down.
I found a specialised optician who we saw every 2 weeks who gave him exercises to do daily and six months in it was making a difference and a year in he was so much better.
So what I would suggest:
Check the obvious things first.
Get an eye test.
Get a hearing test.
Then, if both of those are ok, try for an assessment with an educational psychologist.
Some schools buy in time with an educational psych so you may be able to get her seen through school. If not, it will be private, and that does cost money.
If you can't afford that, try to get a tutor for maths and English with experience of kids with SEN/kids with low attainment.