UCAS is UCAS - whatever email address you use for UCAS will be forwarded on to the universities you apply to.
If you've signed up for open days etc with a different email address then you *might* get duplicate spam to both addresses.
I would recommend using your formal email address for UCAS (especially because admissions staff can SEE your email address so if you have something "funny" then it doesn't look great). Make sure it is an email address that you will still have access to in August next year - a lot of applicants use a school/college email address only to find they can no longer access their emails from UCAS/universities after the end of their exams.