Whats the 1.5? From Imperial website;
'The minimum entry requirements for this course are three A-levels, including Chemistry and/or Biology and one science or mathematics subject, and one additional subject at AS-level. If either Chemistry or Biology is offered alone at A-level, then the other is required at AS-level with at least a grade B.
Our normal offer is for grades AAA in three A-levels and B in the AS-level. For candidates offering four A-levels our normal offer is AAAC. Vocational A and AS-levels are not acceptable and general studies will not be accepted at any level.
All applicants must have the following subjects at GCSE level, at grades AAABB or above (in any order): Biology (or Human Biology), Chemistry, English Language, Mathematics (or Additional Mathematics or Statistics), Physics. The Science double award may substitute all sciences at GCSE.'
The fact that their offer for 4 A levels is 'AAAC' means that they probably don't care that much about your fourth A level. Plus the sentence before that. It's from this webpage;
http://www3.imperial.ac.uk/ugprospectus/facultiesanddepartments/medicine/mbbsmedicine/entryrequirements