Too late to defer University place?

I got accepted into my first choice uni and am due to start this September (in three weeks) but with the reality of getting the reading list/thinking about packing I have started to feel that I'm not ready to go to uni this year. I'd like to work, get some experience and maybe do some volunteering but I'm not planning any trips abroad. I definitely want to go to this uni next year to do the same course though, I just don't feel ready right now.

Given that I'm due to start in three weeks and I've already started the online pre-registration process, is it too late to ask the uni to defer my place? And what would I need to say to present it in a positive light? Does it sound bad to say that I don't feel ready? Also, I didn't quite meet the grades of my offer but they let me in anyway - would this affect their decision whether to let me defer or not?

Thanks in advance for any help/info