How do I defer a year and get a placement in a school?

I am hoping to become a history teacher in the future and have applied for university and made my firm (Manchester Met) and insurance (Lincoln) choices. It wasn't until after I did this that I started to consider deferring for a year; I don't feel completely ready for uni yet, I feel like I need a year to gain some experience and do some travelling whilst saving up some money for Uni as I don't want to work whilst I am studying for my A-Levels. I'm not sure on exactly how to defer and if its even an option now that I've made my choices, or if it would be possible to work in a school as a teaching assistant with no qualifications other than my GCSEs and A-Level's. I don't know how I can get into working in a school, and when/how I can approach any of these issues. Help!