Masterfoods have, for some inexplicable reason, decided to limit their market by making most of their chocolate unsuitable for vegetarians by using whey from animal rennet!
I cannot understand their logic, but presumably this sort of whey is a lot cheaper, as decisions like this usually end up being about money.
Anyway, you can see a list of what is and isn't veggie-friendly here (it includes things like Mars, Snickers, Maltesers, Galaxy, Bounty, Milky Way...). You can email them to tell them what you think here.
As I already boycott Nestle products due to their dodgy approach to baby milk, I now can't/won't eat most mainstream chocolate (with the exception, I think, of Cadburys?). Oh well, thank goodness for Green & Black's!