Hi Ellie
I did exactly what you're proposing to do at university, and Spacial is right, talk to your tutor. They won't turn you away and you won't have any problems going back. Deferring for a year (or 2 in my case) isn't the same as leaving uni, you are still enrolled, you've just deferred for however long. In fact I'm almost 100% certain that students are allowed to defer for up to two years once they've enrolled on a course, either before like a gap year, or during which is what I did. You seriously won't have any problems there.
As for what to say, that is entirely your choice...I used money and my complete lack of it as an excuse, the actual reason I deferred was because I'd started doing lots and lots and lots of drugs and had fallen in with a social scene that wasn't so judgemental about my female character traits, even though I was still keeping the GID completely secret at the time. As you can imagine that was probably more addictive than the drugs and I spent spent about a year focusing all my attention on my social life to the detriment of everything else. I didn't say any of that to my tutor though. all they really needed to know was that I wanted to defer, not the microscopic detail as to why.
Hope this helps and if you want to talk more, please PM me
Helena x