Few things:
1. I am not intimately familiar with how London hospitals organise volunteering (I certainly didn't have to pledge allegiance when trying to find shadowing work in Nottingham) but I've heard it mentioned a few times in threads and the consensus is pretty much just tell them you're considering applying to Kings and that's fine. I personally think it's ridiculous that they expect you to decide which medical school you want to apply to before you even know if you want to study medicine.
2. The comment about "people that volunteer not doing very well at A-Levels" is complete crap. Every single person that has gotten into medical school in the past, say, 10~ years has some kind of voluntary work. Does that mean they've all not done very well at their A-Levels...?
3. The comment about hospice volunteering not being very good is complete crap. It (obviously) gives you no insight into the life of a doctor etc. but that isn't the aim of voluntary work. Voluntary work aims to give you a hands-on experience in actually caring for people. Getting into hard situations - emotionally or physically - and dealing with those feelings. Shadowing work gives you absolutely no idea what you're going to "feel" when a patient dies, for example. My voluntary work was millions and millions of times more valuable in deciding that medicine was right for me than my NHS shadowing. In summary, shadowing work and voluntary work are two completely different things - you can't compare them, but they're both equally important.
4. This course won't give you any "higher" chance of interview as any other chap doing shadowing work in any other hospital. It's down to how to how well you communicate what you saw, the qualities of the doctors and how you have these qualities etc. in your personal statement that will get you the interview.
That lady you spoke to has no idea what she's talking about, but a 5 day work experience placement is a brilliant opportunity for anybody.