The similiarities end there because while Haddon is a writing a family story about a specific incident, Boyce is writing about a large incident that happens to a specific family. The difference is notable because Boyce makes a mistake that I don't think Haddon would fall into.
The mistake Boyce makes is that he uses a religious group to make a point, but he gets a whole bunch of facts wrong (and they're not the narrator's wrong facts, they are the author's wrong facts). And unfortunately, the group he picked is a group that I happen to know something about, being one of them, which is Latter-Day Saints.
Boyce is using the group to make a point. The story is about two brothers who find a sack of pounds two weeks before Britain goes over to the Euro. And they have to figure out what to do with it (other than give it back). The younger brother, who is fascinated by Saints (Catholic, not Mormon), wants to do something good, which sounds more sanctimonious than it is. And the older brother just wants to dispose of the money successfully. But what they discover is (1) inflation is a problem since if you offer ten bucks to a fellow student to get your lunch for you, they're going to demand more for anything harder; (2) everybody wants money, not just greedy capitalists but well-meaning charities as well. Money is complicated. Money can do good. Inflation is bad. It's really a very clever book.
They also discover that people become unprincipled over money. Which is where the Latter-Day Saints come in. They are used as the token religious group that says it is anti-money, until you offer them some when they will turn into liars.
Now, Boyce's treatment of this part of the book is very light, and I'm not particularly opposed to his use of Latter-Day Saints. It's just, he got so many details wrong.
First of all, he describes the Latter-Day Saints as wearing white shirts and not owning anything. So, it sounds like he is describing missionaries, especially since he throws in bits about them being young and talking with different accents. And they ride bikes too. There's only three of them, but hey, maybe there's only three (rather than four; pairs of two) in England. The narrator gets the idea that the Saints live in some kind of commune (which is sort of accurate) which is why he gives them money.
In other words, from the narrator's point of view, it makes perfect sense that he would think what he does and make the assumptions that he does, and minor details, like one of the Mormons, Eli (where are their nametages?), coming over on his own to install a security camera in the kid's house shouldn't be that big a deal. And the fact that they carry briefcases to the laundramat. Do missionaries in England carry briefcases? Anyway, details-semtails, who cares?
Except the author does know enough to have the narrator go to www.latterdaysaints.org as a link (it gets you to www.mormon.org.) and also for the narrator to learn about the Mountain Meadows Massacre. Except that (1) you wouldn't learn about the Mountain Meadows Massacre on www.mormon.org (although you probably could at the Church History site on www.lds.org if you searched hard enough) and (2) www.mormon.org does not refer to Joseph Smith as wearing "spectacles" (in any way; I checked) when he translated the Book of Mormon (although some Mormons do believe that).
In other words, the kid is bright enough to find out about "spectacles" and the Mountain Meadows Massacre but too dumb to figure out that these are Mormon missionaries.
Personally, I don't think they are suppose to be Mormon missionaries, which is too bad because I could see a house of Mormon missionaries taking 3,000 pounds and buying VCRs and stuff and not telling anyone, including their Mission President. I actually think that is a big hoot of an idea. But, as I say, I don't think that's what the author is aiming for. For the sake of theme, he wanted some ultra-religious cult, the kind of organization where "they all lived together in the same house and in the daytime they all went off together like the twelve Apostles" and where "they walk[ed] in a line . . . as they passed you they would each nod at you, one after the other, like ducks in a shooting gallery" (which is cute writing, but I've never in my life seen 20 year Mormon missionaries act like that, even the really uptight ones), but he didn't actually know any cults, so instead of inventing one, he picked a random weirdo religious group out of the air . . . and didn't even bother to research his own link. It's pretty obvious that the kid's internet research is whatever little bits of info Boyce actually knows about Mormons, and he lays out those bits in the text so that he can have the kid say, "It was all a bit literal," which again, is a fairly funny remark, except that the kid means it negatively and this is a kid who is so literal that he buys a whole bunch of birds from a pet shop and releases them like Francis of Assisi. But the kid isn't negative about Francis of Assisi so why is he negative about Mormon theology? Unless the author just wanted him to be negative about Mormon theology.
In other words, after awhile, it started feeling a bit anti-Mormon. It doesn't quite cross the line, I still recommend the book, but as my parents used to say about anti-Mormon pamphlets at the Hill Cumorah Pageant, "I could write better stuff than this."
(Of course, the biggest problem, is that Mormons aren't even vaguely anti-money--collectively, at least--those corrupt American capitalists--which makes them a darn funny group to use in this respect.)