Preliminary comment: Thinking in terms of ranking is usually not a good idea. You should think of the best place you can be for your specific area of interest. That may be the best groups in a field and yes, they may be at a top-ranked university, but then again, they may be further down that ranking list. Just generically aiming for high-ranked places is not going to advance your career unless you can be motivated by prestige only and you do not care what you do.
Now to your main question: can you ask for MSc studentship only? You can ask, but it is not a good move. If you get offered a PhD and downgrade your acceptance to a MSc, it shows limited committment and interest on your side (and correctly so, given your thinking in terms of ranking), and it's a waste of time by the prof to educate you in a field, only to see you leave.
In fact, many profs would like to upgrade some of their MSc students to PhDs when they are good. The opposite direction is not attractive unless the student is weak.