It's perfectly possible to take a gap year for medicine. You can apply for deferred entry while in U6th (i.e. the same time as everyone else) or wait and apply the following year. If your predicted grades are up to scratch I'd recommend going for deferred entry, because that way if you don't get in you still have another shot the next year without having to change any plans.
There isn't a set amount of work experience that is "enough" for medicine. What you want is an insight into life as a doctor, to help you decide whether it's right for you or not. Generally trying to get some time in hospital and some time in GP is ideal, but admissions tutors know this isn't possible for everyone. On top of that you should be looking for some long-term volunteering work in a caring capacity (care home, hospice, hospital volunteering etc) to help develop relevant skills for medicine.
If you want to do medicine, chemistry A-level is fairly non-negotiable, I'm afraid. I loved mine so can't really compare experiences. As for GCSEs, nobody's quite sure what requirements will be with the new grades, but if you were to translate those directly into A*/A/B, they'd be ok for most unis except the most GCSE-heavy ones.