Generally all universities accept mature students, although some are more facilitating of this than others. As you're not a "mature" student by just taking an additional year to cover subjects necessary to the course that you hadn't done before, this isn't really relevant though.
I would suggest contacting the universities in question with your plan to see if they will accept that, as some may not accept the A-levels taken in a second exam session, even if they're different subejcts. They may require you to do 3 full subjects for the session, for example...You may want to consider either a course with a foundation year, or an Access Course. In the latter case you should contact unis you're interested to apply to, to ensure the qualification is acceptable to entry for the degree of interest. The former especially is quite a common route and offered by many universities in relevant courses - for example Southampton, Lougborough, and Manchester all have a number of such foundation years last I knew.
Imperial itself does not offer any such foundation courses, and is notoriously unhelpful to actual mature applicants (i.e. those who are above the age of 21 and/or had a gap in their education). They have not accepted more than 5 mature student to any given faculty/department (covering all degrees in that faculty) in the last 5 years with I believe the occasional exception of Maths, and frequently don't take on any mature students - despite having mature student applications in every year to every faculty. Thus, it is fairly unlikely, in any case. I do believe Biochemistry and Chemistry are among the faculties/departments that have taken mature students with some level of consistency over the past 5 years though, which may be relevant to your interests.
Ultimately you should contact the universities before embarking on any plans to avoid potential disappointment.