When we used Atom a few years ago, we found it really inconsistent - the same questions cropping up repeatedly, and scores all over the place. It's been discussed on here a lot that Atom find ways to increase your insecurity as parents and keep you paying. We only did it for a short while and gave up, as we felt there were more useful resources.
We found CGP and S&S books much better than Bond. Exam Papers Plus are good resources but very expensive - if you can team up with another parent (or two) and split the cost, it's less painful. (That's what we did - the files are password protected but you just print them off for each other, or share the passwords). Pretest Plus is good for specific online tests like the London Consortium but again very expensive, so we used them sparingly - just one of each relevant test (that we couldn't get elsewhere) for familiarisation.
There's a really good website with lots of past papers from various schools - no answer sheets, so you have to mark them yourself, but useful for practice questions and timing:
www.11plusguide.com/11-plus-papers-books/free-11-plus-papers/free-sample-11-plus-independent-school-papers/
And closer to the exams, make sure they're really familiar with the past papers for each school they're sitting (available on the school's website).
Outside of that, I'd say make sure they're reading a lot - not just because it's good for their vocab and story understanding or general knowledge, but just because it's good for them, full stop.
As regards tutoring, I would be amazed if there were any prep schools in SW London without a single child being tutored in Y5/6. Even if parents aren't talking about it (and they rarely do), it'll be happening. Which isn't to say that every child needs to be tutored - they don't. But it is widespread. And not just for those aiming for top tier schools, in my experience.
Good luck!