I think it will depend on which BTEC course you are doing
e.g. Manchester will accept National Extended Diploma (i.e. the one worth 3 A levels) and require D*D*D - DDD
or the National Diploma or Certificate D*D* - DD
plus all of them require Maths and Physics A-level at A*/A
So it depends on the size of the BTEC qualification you are doing.
You need to go to uni websites and look at their requirements. For some you may need to email the admissions secretary to ask about the specific course you are taking.