As I've said before, I'm pretty close to 100% certain that they are NOT there to pretend to be tourists. It would perfectly fit in with the Harry Potter Universe for there to be people like this in London. Specifically in the Goblet of Fire (which if I'm not mistaken is when the first Harry Potter section is meant to be) when you had lots of witches and wizards descending on an area trying to fit in with "Muggles" but failing miserably to do so. Their fascination with a camera makes perfect sense within the books as well, as Arthur Weasley shows a particular fascination with "Muggle" technology. When looking at them in this manner, as curious witches and wizards that are trying (and failing) to fit in with the "Muggle" world, as opposed to annoying tourists, they're nowhere near as annoying.