While the Army has no "legal" right to intervene, it doesn't mean that they won't intervene if it makes Beijing happy for them to intervene.
To be honest, the US is just blabbing a lot of noise and won't do anything when it comes to game-time. They cannot. China effectively owns the US through debt.
The UK.... I just wonder if there's anything in the 1997 handover statement that would allow the UK to at least publicity shame Beijing into making the election "free" from Beijing's meddling. Thing is, Beijing could just have simply just allowed the sense of free speech and freedom to choose as well as vote for whomever candidate they wanted to choose. But what they could have done is to funnel money to only the supporters of Beijing themselves and just make it much harder for those who were not pally with Beijing to campaign through secret taxes or bribes to other businesses/political members.
Beijing as a country is still corrupt when it comes to the people in power so I cannot see why they didn't do it that way.
EDIT:
Just saw this on the BBC running news commentary:
"10:49:
The White House
has responded to those who have signed a petition to the US government about the Hong Kong protests. It says the US believes the "legitimacy of the chief executive will be greatly enhanced if the Basic Law's ultimate aim of selection of the chief executive by universal suffrage is fulfilled and if the election provides the people of Hong Kong a genuine choice of candidates"
Well, why did the American public even think about creating a petition to the US Government to weigh in on it? The US doesn't really have any particular links to HK apart from economic issues. It would have been a more powerful message if the UK government got petitioned to comment on it, which they had already done that.
I also just quickly looked at the link to the petition itself. Well, it certainly seems like someone doesn't quite understand how the situation is like here. Tienanmen, when you look at the history of it, was only special because it was captured by the media with photography during that time. During that time, there were even more violent crackdown on protests. At the time, the UK and the US didn't do anything to safeguard the freedom of the people, mainly because it was a Chinese issue that at the time, could only be sorted out by the Chinese.
The main issue now is that there is a disparity between what Beijing wants for the rest of the country and to get HK to agree to, but HK residents doesn't want to submit to the rules and regulations which would lose their freedom to protest and keep their Special Administrative Region system going.
Yes, there could be an issue of armed response to the situation, however that is not likely to actually happen as China knows the rest of the world is looking at Hong Kong itself. It has to tread a relatively fine political line.