This Commonwealth of Nations troper would like to know how in God's name Jack Ryan acquired a knighthood ... no, scratch that, why he's called by the title 'Sir John' in the United Kingdom, even though he can't accept the knighthood at all, despite Clancy's Hand Wave ... no, scratch that, what I really want to know is how Jack Ryan gets to be so bloody rude to a Captain Ersatz of Prince Charles and get away with it.
In fairness, Jack knew he was being an ass, and knew he'd be hip deep in shit if it backfired, but he was basically trying to use shock tactics to prevent said Captain Ersatz from slipping into a Heroic BSoD because of his failed I Just Want to Be Badass moment. It winds up working out well enough, but he does get called out for the rudeness later on, albeit comically.
As for how he acquired a knighthood in the first place — well, as mentioned in the story itself, its been a long while since an heir to the throne was single-handedly saved from certain death under such circumstances, and the Crown's dignity sort of requires an appropriate reward to be handed out when someone does.
Also, it's acknowledged in the book that while the knighthood is technically honorary, as a US citizen can't hold one under US law, it's treated as being a legitimate, full one as far as personal interactions in the UK are concerned, as a show of respect for Ryan. See also the knighthoods given to Nimitz, Eisenhower, and some other US flag officers of major commands during World War 2, plus other US citizens in Real Life who've been knighted.
If we're going by commonly accepted US law (or at least the current Constitutional interpretation), then Americans are allowed to accept knighthoods or titles from foreign dignitaries, but are not allowed to accept any of the more tangible benefits that come with such grants, like lands or incomes. So it's okay for Jack Ryan to accept a knighthood, maybe with a nice framed commission or plaque. But if Her Majesty were to offer him a lordship for whatever reason, then we'd be running into trouble.
She could technically only bestow the title and not the positions on him. Or she could give him an unused title.
Miller's highway ambush
Was it really easiest for Miller to ambush Cathy Ryan on the highway? I'd say that's kind of risky, given that A) a bunch of motorists are right behind him who could tell the police that right before the shooting they saw a brown windowless van driving erratically, going at least 30 MPH over the speed limit as though he were in a real hurry to get somewhere, and B) there's at least one motorist (the driver of that black car that briefly comes between Cathy's car and Miller's van as Miller is preparing to open fire, and momentarily stalls him) who is close enough to practically describe Miller in detail to a sketch artist if he ever was questioned.
Miller presumably wanted to ambush Cathy's car in the quieter residential area, but the unexpected crossing guard ensured he had to catch up to her.
Furthermore, what did Miller and his driver do with the van? Did they torch it or did they just abandon it as soon as possible?
The book explains this: The reason for the highway attack was because Cathy had gotten in the habit of taking multiple routes to and from work, but since she had to drop off Sally at the day care center, there was one place she always went to on her route, and thus one place they could be guaranteed an opportunity to start a pursuit. The ULA wanted the attack to be easily witnessed, because the whole point of the mission (In the eyes of Miller's superior, anyway), was to make Irish terrorists unpopular in the US, thus cutting off the IRA's gun money and leaving them vulnerable for the ULA takeover (They had a different funding source). And the van (Which had no fingerprints because everyone in it was wearing gloves) was driven to a local mall, left in the parking lot, and then the people in it entered the mall and exited in a different lot where another car was waiting.