This seems to be a common way of doing it. I wouldn't worry other than to make sure it says something about refunds, if you fail to complete the entire course, in your contract.
Schools are usually approachable about different payment terms. Remember that with such a lot of money involved almost everyone has to have some sort of string pulling happen somewhere just to get on the course. Not many self sponsored students can front all up in one go. However, it may cost you more if you pay in installments or make non-standard arrangements. I suppose this is to cover their lost interest since the money isn't with them for as long.
I don't have any personal experience with Oxford, but I wouldn't worry about voicing your concerns to them. I'm sure they deal with things like this all the time.
cheers,
-abracadabra