It might surprise you than you dont need A level Comp Sci to do a degree in Comp Sci - Maths is far more important and is often the only specified subject.
So, the important thing is that any other subjects you take are subjects that you will enjoy studying and where you feel confident of getting top grades. If Chem is that subject, then do it.
Do not assume that Physics, or any other subject, is 'required' or that this will help your application, unless they actually say that on the Uni course page. Unis must tell applicants the truth about what they are looking for each course - there is no 'secret information' that only special people know about. So, dont assume anything about preferred subjects that is not stated on the course pages, and dont listen to any teachers that say things like 'All Unis prefer xxx' - they are usually wrong.