This seems like something that would take some amount of work to fix in case the author did care. Problem with speech pattern differences is that you have no idea they're there if you're not familiar with the target speech pattern, so it's not like regular fact checking where you can generally tell when you're dealing with something tricky. I'm not terribly familiar with spoken English and had no idea about incongruent Americanese in the lines, though I can't think of anyone sounding very British either now that you mention it. The most straightforward fix would seem to be to run the dialogue through a native British English speaking editor, and that's a bit heavyweight for a fanfic.
In fanfiction, the problem is solved (if the writer cares) collaboratively-- American writers trying to do British English well is such a common problem that the proof-reading and copy-editing has a name: Britpicking. I assume that most of that is done by native speakers.
The problems can be subtle. I was shocked to find that modern British English doesn't include "gotten". How do they make it through the day without such a useful word?
And I'm not going to mention the book because the author's a friend, but she writes excellent British English. Wh...
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.