I did Level 3 BTEC Extended Diploma in Applied Science, and it is quite difficult. You'll have around 12-13 units and 3-4 assignments for each unit with multiple pass, merit, and distinction criteria you need to meet to get a distinction. If you get a merit/pass in one assignment of a unit you will be capped at a merit/pass for every assignment in that unit. If you have good time management and don't leave assignments until day before submission you should be fine with this, though. Exams were in January for both years of my BTEC, so you'll also need good time management for that (if exams go ahead this upcoming January) as you'll need to revise and work on multiple assignments. (For reference, I had to revise for exams and complete 8 assignments in semester 1 of year 1 (Sept - Jan)).
Not sure if it's still a requirement, but everyone had to complete 40 hours of unpaid work experience each year of the BTEC in a science-related industry. This may not be a requirement anymore, just making you aware.
An important thing about BTECs and entry into university is that universities usually ask for your BTEC to be accompanied by an A Level, so A Level Biology if you're wanting to study biology at university, or they'll want you to have completed specific units in your BTEC.
If you're wanting to go into science at university, the A Level in Business will unlikely be of much help unless the universities you're applying to accept BTEC accompanied with an A Level not relevant to the course you're studying. Some do, but it's not many.
Hope that helps