That's certainly not the case in my experience. All of our children go onto independent senior schools and the reputation with senior school heads is very important. If you sent someone who turned out to be a dud, the senior school will be less accommodating in the future with your borderline kids, 'late developers', and those with extenuating circumstances. The headteacher's reference is an important part of the process, alongside the test results, so it is vitally important to keep onside.
The key thing is in getting the right kids into the right schools for them. That means only considering suitable schools in the first place, and the prep school can do some amount of pretesting to make sure that they are in the right NFER range etc.
I certainly wouldn't call a 30-minute session on VR/NVR, where all the children do is go through their Bond papers on their own as cramming (a non prep school child can do this at home for far more than 30 minutes a week if they like).
In my experience, all the pressure for unsuitable senior schools comes from the parents, not from the prep school.