No you certainly don't have to stay 28 days. Although you will be on the books (and be paid) for months afterwards you will be out of the front gates and your PVR will be accepted a few days from your last interview at DIOT and the world's your oyster.
If the head shed see that you really don't want to be a part of the service, your reason to leave is sound and thought through they will let you go quickly, and possibly even quietly express thier support for you.
But unless you have been soured by previous forces experience or have given it a lot of thought there's really no reason why you shouldn't give it a pop for 28 days at least if you've been accepted.
There are plenty of people who would give their right arm to be going to Cranwell like you, but at the end of the day if you do decide to leave it's your future that is the most important thing, not how lucky you are to have got through OASC.
I would suggest joining the Reserves before taking up a Commission, then you would be better positioned to make the decision of whether today's armed forces are for you. But given the short time scale this won't be possible for you!
All the best whatever you decide.