I am currently studying pharmacology at Uni of Liverpool and I was a BTEC student. It is an AAB A-level entry requirement course. I have the Applied science Extended BTEC pre 2017 D*D*D* diploma with all the 'necessary' biology chemistry units.
I'll be blunt - I am struggling.
The BTEC just doesn't contain anywhere near the necessary content needed to understand the basics of my modules. My BTEC didn't teach me anything about calculus, enzyme kinetics, membrane potentials, chi-squared tests; the list goes on and on. I'm currently paying Β£30/hour for a tutor as my mathematics and chemistry is just not up to scratch. The majority of A-level students have no problems with any of these subjects.
Another thing is exams. I did terrible in my exams which is probably due to my lack of understanding, but I believe not taking any exams for the BTEC definitely had a part. Although I did get one first, I barely scraped the hard science exams with 40-50% each. Keep in mind, I'm not saying you cannot do these courses/exams without A-levels, but I am saying that you are going to be dealing with a lot more pressure and stress.
All in all, yes, the BTEC is much easier in comparison to A levels but it isn't worth it as you are not prepared properly for degree level and will most likely need to do catchup. This isn't ideal as STEM courses in particular are not easy. For example, we are in 9AM-3/4PM 4 days a week as well as labs, online tests, cycles (which are coursework set by your academic advisor) etc etc so the workload is intense enough already. Obviously each university is different but I'm just giving you a general idea.
Let me know if you guys have any questions or whatever. I'm not sure why I wrote this I just thought I'd post it as I'm sure this will get asked now and again.