As above, you may need a separate A level alongside your BTEC for some courses - Radiography for instance relies on Physics and a 'general science' BTEC may not cover this in enough depth.
However, there are exceptions - especially if you have taken the 'right' units within your Applied Science BTEC.
As an example, Bath will accept BTEC Level 3 National Extended Diploma (RQF) in Applied Science for its Chemistry degrees - grades DDD - but you must be studying the following units : Applications of Organic Chemistry, Applications of Inorganic Chemistry, Industrial Chemical Reactions and preferably, Practical Chemical Analysis. For Biology, Bath will also accept this BTEC, but again only including specific units.