Harry Potter! In this scene, Mr. Vernon is irritated and confused by the appearance of people in cloaks. He doesn't want to believe that something strange is going on, because he prides himself on his normalcy, so he is trying to make excuses to himself so that he can believe everything is normal. Thus, he thinks to himself, "these cloaked people are probably just collecting for a charity, and are dressed strangely in order to attract attention for their cause." However, this excuse does not convince him because they do not have collecting tins, which are boxes that people collecting for charity put the money they collect into. So no, it's not an idiom, it's a box to put money in for charity.

It's not an idiom. The writer has simply given you a little idea and allowed you to deduce the rest. The collecting tin is a reference to people asking for donations on the street. To add to the mystery of the groups, they DON'T have collecting tins. A collecting tin would mean these people are therefore collecting money, which may explain the uneasiness the character feels (ie. the same uneasiness the character might feel if asked for money in public).

I don't think it is an idiom. The passage is talking about a plate that beggars might use when soliciting money.