Firstly, let me make the obvious comment that this is a tragedy and state that I envy the US position in that the people who did this in most states would be done to death for it.
With regards to arming Police, its important to start off with knowing who is actually armed:
i) the Armed Response Units - every force (in the UK there are around 40-50) will have highly trained officers who carry all the time they are on duty and who should respond to incidents where firearm or other weapon is involved. Certain forces, especially the Met but also Greater Manchester Police, Avon and Somerset Police, Merseyside and West Midlands Police, have rather more of these because of their specific problems. In certain areas of those forces (eg: Lambeth, Hackney, Brent, Moss-side, Toxeth) there will be armed response in the area 24/7. In London these are the white or silver cars you see driving around;
ii) Diplomatic Protection - the Met has a large unit responsible for the exterior protection of Embassies, some Government buildings (especially Downing Street) and patrols between those premises. These are the red Police cars most of you have seen in London;
iii) Royalty Protection - responsible for protection of Royal premises;
iv) Special Branch - parts of SB routinely carry, either in investigations (allegedly) or in their roles as close protection officers (what your Secret Service does);
v) Specialist Operations - some of the more arcane squads allegedly are armed, in some cases heavily so. This includes our version of your SWAT teams, snipers etc.
vi) Aviation Security Units - guard airport premises.
Its estimated that one in twenty officers is armed all the time when they are on duty; this proportion increases when you add the ex-members of those units and former services personnel (of which there are quite a lot), and the rumour is that the Met at least has the firearms to arm all 30,000 odd officers, so they could concievably increase the numbers of armed officers quite quickly - the latest figure I've heard is one in ten.
Historically, at the very start of the modern Police in this country the Old Bill were armed with pistols and cutlasses, that fell out of favour and by the turn of the century there were so few armed Police who were able to respond to incidents that they were forced to borrow firearms from members of the public when chasing two cop-killers. The situation fluctuated given the two world wars and austerity up until the situation was radically changed; ironically enough with the loss of the death penalty.
Police had always been killed by criminals in the line of duty, prior to 1966 they had (almost) always suffered the extreme punishment for it, even in circumstances that modern, less enlightened, society frowns upon (Derek Bentley was hanged for the murder of Pc Sidney Miles for saying "Let him have it, Chris" to a gunman). The abolition of the death penalty in 1965 however meant that when the next main incident occured - the shooting of three unarmed Policemen by criminals in Fulham, London in 1966 - the Government could not rely that the guilty would be executed (despite the massive public outcry for its restoration, popular support for the death penalty which remains to this day) and so, fearing a political backlash, they began to draw up the Firearms Act 1968, which remains the starting point for all firearms legislation in England and Wales.
It also sets a precedent in that successive governments, of whatever hue, usually respond to incidents by legislation "demonstrating" that they are not soft on crime - Hungerford and Dunblane were shortly followed by new legislation which further "tightened" firearms control. The other demand, which almost always follows incidents of this kind (as well as terrorist attacks, child-murders etc) for the restoration of the death penalty always falls on deaf ears, despite (as mentioned above) massive public support, as every main party and most of the smaller ones oppose it in principle.
The number of armed Police slowly rises in the intervening period from then to now, what with armed criminality, the increasing drug problem and several phases of terrorist attacks, the most lethal being that of the IRA. Throughout, however, the bulk of the Police in this country have been unarmed officers; ironically enough because there are so many of us it is almost always we that come into contact with the various armed criminals first (I'm unaware of any death of an armed officer in England and Wales in recent times) with the attendant risks involved.
Despite this the majority of my colleagues at least (and, if polls are to be believed, in the country as a whole), do not support arming all police with firearms. It would be a radical change that would effect each and every part of an officers job, especially in the way we deal with criminals (British Police are a lot more tactile than yours; where I see your officers pointing their gun at criminals and subduing them that way, we are more likely to (if theyre unarmed) to bundle them over). That said, there is far more demand for less-lethal weapons, like the tazer, to be issued to all-ranks; our CS does not work on everyone (it doesnt work on me) and the ASP baton is not as threatening as it should be (it doesnt deter people from having a go). In addition, the public would not expect it and more than a few people would be shot early on in circumstances where they would not be shot before; I can think of at least three people I would have shot who I was able to restrain without a firearm, even though I was in fear enough to use one if I had had it. Plus, as Iain notes, there is a whole issue over hypocriticalness where we would state that we had to be armed wheras society couldnt - the theory that the Bobby is merely a citizen in uniform, paid to observe full-time the responsibilities that are incumbent on all citizens, would be finally killed off (if it isnt already)
Of course, as is usual, the media comments are rubbish; there would be no "escalation" if all Police were armed because criminals (amazingly) do not play by the rules.
