You do engineering core units with electives taken up by science. Should you decide to transfer to science, you can accredit up to 48cp from other faculties to count towards your science degree. Hence the latest you can leave is about the end of the second year, pending on subjects, but you would have missed out on most if not all of the prerequisites for third year science and some even for second year science subjects, and you might have to spend an extra year.
Should you choose to do flexible first year into biomed, in first year you have maths and chem in first semester, and the addition of biology in second. You still need another 6cp of junior science subjects should you wish to transfer to just a science degree and qualify for it.
The latest you can transfer without additional hassle is really at the end of semester one, unless you're willing to stay in engineering and just do science subjects and transfer later, or overload.