I'm afraid that, the more I think about this, the less I think it is possible to fill the perceived gap how you are describing. My response has turned out to be a bit negative, but I think it's worth sharing if only to prompt some further thoughts from you and from others.
There are already online offerings that include learning resources and practice materials for both SATs and 11 plus. Obviously they come at a cost, but I would expect these to have fulfilled your brief already. Are you hoping to challenge the likes of Atom and CGP+?
From your second comment - I don't think a single theory book and workbook (with online tracking and tests) could meet the needs of all parents helping their children to prepare for maths test for both the 11 plus and SATs.
Children of all abilities take SATs. Even if one ignores the fact that SATs home prep ought to be unnecessary as SATs are as much about the school as the individual students, the range of abilities is too wide for one theory book to cover adequately.
For the 11 plus, the maths requirements vary between schools/areas and the standard is generally higher than for SATs, and for some specific schools it is considerably higher.
These books would have to be enormous, and parents would need to sift through all that content to find the relevant sections - which is effectively what they do anyway when looking for suitable material for their children.
Within the more affordable paper materials you can simply choose books for a lower or higher year group or go for the stretch materials.
If a maths concept is alien to a parent, it's easy to use an internet search for online explanations/videos.
The other people that have responded so far have mentioned a range of resources they have used, depending on the needs of their children. I don't think it's very difficult to find material at a suitable level for your child if you are an engaged parent.
I dare say there's space in the market for another set of resources though.