I'm an a-level student myself, and have just completed Biology a2. My friends take maths, chemistry and physics, and laugh at biology for the same reasons. I think that because it is more an observational and life science as opposed to a physical science like chem or physics, people see it as 'easy'. It is certainly not the case, perhaps the exams are a bit different?. Trust me, if you can achieve the grades to study biology at Imperial, or at any university for that matter, then you should feel smart. I found biology a-level so hard, and have huge respect for people who study it at university.
Btw, if anyone says that it is easy, ask them to sit the exams with you, and then see if they still stand by their comments. That' what I did with my friend who did maths, chem and physics. He got a D in biology exam, and A's in all the other subjects. Easy? I think not :P