As long as the grades you receive are the minimum or above of what the colleges require you should be fine. If the sponsor has higher requirements than a college requires and states your grades have to be at xxxxx level when they make an offer then they could withdraw it. To be able to go to the colleges you have to satisfy their minimum requirements the sponsors have no way of changing that it's out of their hands. If you are down to do an FD but your grades allow you to do a HND instead they might still keep the offer open.
Regardless, work as hard as you can on the exams you have to do, some times people don't find a cadetship is quite what they expected and leave, at that point you will need the best grades you can achieve to go and do something else.
We all know this last year of exams is tiresome and your fed up, but it's only a few weeks and it will all be over! Good luck!