Can I join sixth form a year late?

Basically, I'm in year 11 now and next year instead of getting straight to sixth form, I want to do a year-long German exchange immersion program (with CIEE, i'll stay with a host-family and go to a local high school and stuff), to become basically fluent and experience something new, ect..
However, when I return I still want go to a proper sixth form (I don't want to go to a collage) and I'm worried i'll be too old and no where will accept me in at year 12. I will be 17-19, instead of 16-18. Is that allowed?
And also, thinking ahead if something terrible did happen during my sixth form years, then what am I still allowed a resit year? A 'second gap-year' if I don't get into a good uni first time round?


Btw, i want to take F maths, maths, physics, german and possibly computing (pending how well i can teach myself maths before hand and how good i get at german) and then do some sort of engineering at uni.


Ok, so thats a lot of questions but any advice would be amazing
Thanks