Eeek! I've got Piggy-Cold* you know. Will go through HOTYH's bits through the course of today. A couple of initial thoughts, though:
1) British-English is slowly conforming to US English. Your wonderful author Bill Bryson (who is also Chancellor of one of my local universities and much-loved aficionado of one of our prettier local cities) has written on this subject and, if memory serves, discusses it in his book, 'Mother Tongue.'
2) HOTYH says that the grammar is poor yet proceeds to examine the grammar forms to try to prove that the writer wasn't necessarily of American provenance. He surely shouldn't take the absence of a comma here or the exclusion on a hyphen there to prove anything - using his analysis, I mean. In fact, hyphens are a huge bugbear here and you find nerdy societies dedicated to restoring certain rules of grammar (eg. The Apostrophe Protection Society).
3) He quotes the provenance of certain expressions and suggests that they are actually as British as they are American since they have common origins. In fact, he is very easily proven wrong: In the 1700s, we said 'gotten.' Now we don't. You do. You may get the odd person saying 'outsmart' but in fact, we refer generally to brightness, cleverness, being on the ball, rather than to smartness and the expression, 'outsmart' is far from common. 'Out-wit' would be my pick** Similarly, most English writers would use 's' in words like 'criticise' but, in fact, there is an entire school of grammar, the Oxford School, which prefers 'z' and you will get some Oxford graduates still using the 'z.'
4) As far as I am aware, pretty much everyone who examined the note bar HOTYH thought that it was American English. Is he suggesting that , with the benefit of their educations on the subject, they don't know what they are talking about?
5) I was discussing how the letter read to me, a 30-something graduate who, through work, spends at least as much time reading American correspondence as she does English. It reads as US English to me. HOTYH thinks not. Ok. Fair play. His view. How do you other guys*** feel about it?
6) He misses the point in picking up expressions like 'Victory!' as being uncommon. They are uncommon everywhere bar secret revolutionary societies in bad films. The writer was trying to mimic what he or she thought would be in a ransom letter.
7) The writer was deliberately trying to sound foreign, as indicated by the risibly misspelt words at the beginning of the letter.
8) More later.
* Got piggy-cold but the oinkment is working. Ho ho ho!
** I love the quote about refusing to engage in a battle of wits with an unarmed man.
*** An example of American idiom being used by a Brit. 'You lot' would be far more common.