From the University of Leeds website:
"AAB at A-level - applicants must study one or more of the following subjects: psychology, geography, mathematics, chemistry, biology, physics, geology, economics, statistics, environmental science or computing.
We also require the following specific GCSE qualifications:
English Language at grade C and Mathematics at grade B plus one of the following science qualifications at grade B: physics, biology and chemistry, science or additional science. We would accept GCSEDA Science (dual award) at grades BC."
Doesn't make any indication as to a fourth AS level, so it would be okay. As you probably know already, Biology is probably the best and most related A level subject to Psychology at University level (apart from Psychology A level as well, of course), so it would be good to at least have AS level in Biology. Then again, if you think you can only achieve a C or D, perhaps it's not worth it.
You could drop it and focus on getting good grades in the other 3 subjects and it sounds like it won't hinder your application for University of Leeds.
As for Leeds Met:
"UCAS Tariff Points: 300 points required. (Minimum 200 from 2 A Levels or equivalent, excluding General Studies. At least 80 of these points must be from a scientific or numerate subject. Subjects can include Psychology, Biology, Mathematics, Physics, Chemistry or Geography.).
If you're applying via UCAS, find out more about how your qualifications fit into the UCAS tariff.
Additional Requirements: GCSE English Language, Mathematics and Science at grade C or above or equivalent."
So basically 200 points has to come from 2 A levels, and then it's saying the extra 100 points have to come from anywhere else more or less, so for you ...you have to get at least 100 points in one of the subjects (B) for definite and then 200 points in the other two in whatever combination of grades that equates.
In short, yes you can drop Biology if you're really struggling and unhappy with it. Your application to these universities will be fine.