What courses do I need to take in college to become immunologist, haematologist etc Watch
Find out more about the University of Kent
Some universities offering some of these degrees (with the exception of Medicine) will accept BTEC degrees - however a BTEC isn't necessarily better than A-levels to prepare for a science degree - in fact it could be worse. University level study is necessarily academic, even in ostensibly "vocational" areas like Engineering, Law, etc. A-levels are designed to prepare students for this - BTECs not necessarily so. It's possible you'll find yourself having to work harder to catch up on some areas of syllabus from a BTEC background (although in theory, courses which accept the BTEC will normally not assume this knowledge, but you'll be learning it for the first time whereas the A-level students are likely covering for a second time) and you may find the lower emphasis on assessment by formal examination means your study habits haven't been developed as well as your A-level counterparts (most degrees are primarily assessed by examination - even in e.g. Pharmacy you'll need to sit and pass examinations to proceed through the various parts of the programmes and become qualified).
As for the US, Pharmacy and Medicine are both graduate degrees only there. US students aren't required follow a programme including their equivalent of A-levels, and study at this level isn't technically required to successfully apply to university in the US. You would theoretically be at the same level as other students, although they may well have done AP and honours classes in relevant subjects which would be more at the level of A-level study. Additionally for the "top" universities (Harvard, CalTech, Stanford etc) most students will have comparable academic background to a full set of A-levels (or even more). In any case, you aren't required to pursue a specific major to continue to medical or pharmacy school - normally you just need to take some specified prerequisite courses (normally chemistry through organic chemistry/biochemistry, first year physics and biology, and usually calculus and sometimes introductory statistics/probability, for medicine - I believe Pharmacy is similar but may not be as extensive). However science majors, especially biology, are common as they contain some or all of the premedical requirements as above as part of the major requirements so there is more flexibility in taking other courses as they want.
However, you cannot just "move" to the US - unless you are a US citizen, you will need to acquire a visa, which means you either need to already have an offer to study at a US university. If you are planning to get a a student visa as an international student, this means you will have extremely limited funding - especially as "need-blind" universities like Harvard are probably unrealistic unless you would be well capable of achieving top grades (A*AA or more) in A-levels anyway, and followed a BTEC on purely philosophical grounds for example (and that may still hinder your application). Otherwise you normally need to be married to a US citizen (and I believe they're working to limit this in some respects, particularly under the current administration there) or usually have a degree and a large chunk of money (like, £20k+ I believe) or already have a job offer and your company is sponsoring you. The latter are not relevant to your situation, and the former is difficult.
If you are a US citizen, then you will still have challenges - going to college in the US is extremely expensive, even for US citizens, and scholarships etc are limited (unless you're at least a county level athlete over here - athletic scholarships are fairly common). It would probably be prudent, both to save money and to ease in to the academic workload of university level study, to begin your studies at a community college - you can normally start first year courses for your intended major and begin completing your distribution requirements.
In essence, it's theoretically possible, but you're taking the most difficult route to your intended objectives (unless of course working as a medical specialist in haematology or immunology is your goal, in which case you're necessarily required to pursue a standard academic course of study realistically). It would be advisable to take A-levels in Biology and Chemistry, and perhaps Maths or Physics as a third option, to pursue this route. You may still be able to transfer to the BTEC if you and your school find that the A-level route isn't working out for you. However the pathways you've identified (even Pharmacy, to an extent) are necessarily academic.