Wouldn't the CAA be more interested in why you're taking the antibiotic?
You could telephone the medical section at LGW and ask them or call your medical officer or even your operations chief? You'll get a different answer from each one so work out the answer you'd like to hear and then make the call to the person most likely to provide that answer?
CAP 715 An Introduction to Aircraft Maintenance Engineering Human Factors for JAR 66 so not 100% relevant but all they had on the subject:
Antibiotics (such as Penicillin and the various mycins and cyclines) may have short term or delayed effects which affect work performance. Their use indicates that a fairly severe infection may well be present and apart from the effects of these substances themselves, the side-effects of the infection will almost always render an individual unfit for work.
This from the FAA for interest only:
FAA Medications, FAA Medical Certification, FAA Approved Medications
So none of that is any help I fear to say. I'm afraid that I never really had the luxury of being able to call in sick, having nearly always flown for measly little airlines that never had standbys.
Good luck in your endeavours, whichever way you want it to go.