For clarification:
My post above was based upon the assumption that you'd already booked a (possibly expensive) holiday before deciding to commence an OU course. I therefore advised what I consider to be the best course of action for you.
However if you've not yet booked your holiday I think that you'd be mad to book it right it at the start of an OU course. You'll almost certainly find the first few months like trying to do two full-time jobs at once (assuming, of course, that you've also got a full-time job to take up your time, as well as the OU course). Adding in an extra pressure around the same time just seems daft!