When I applied to university, I was told that if you're not sure if you want to do the masters or not, then you should apply to the masters initially then change down later if required.
This is because apparently, it's easier to ask student finance to find you for one less year than it is to ask for one more year.
Many of the universities I've seen have identical requirements for both courses, although I think there might be some that ask for a grade higher if you're applying to the masters. In both cases, there are likely to be requirements you have to achieve in exams once you're at university.
At Warwick where I'm at, it's fairly straightforward to switch between the degrees. However, people doing the masters will have certain extra requirements on modules taken in the second and third years so it's better to switch earlier if possible.