I related a story here not long ago when I toured London and the guide mentioned that the police cars with the blaze orange stripe carried the 'Flying Squad' which meant they were armed, while at the same time telling our group that the average London policeman isn't armed like the cops are in America. I pointed out that all I've seen so far are these blaze orange cars and none of the traditional 'bobbies' walking around. The guide sort of hemmed and hawed and mentioned that London was under a heightened terror alert level and that is why armed police were out in force amongst the populace.
So we finally got in front of the US Embassy in Westminster and we saw all these London police walking around with Heckler and Koch MP5s. The guide pointed out that they're carrying 'machine guns'. I looked closer and saw that the Hk MP5s they carried all had the restricted '0-1' ('Safe', 'Fire') "FBI Lower" receiver which meant they weren't full auto machine guns but semi-automatic 9mm carbines without the capability for select-fire. This is how the American FBI issues the Hk MP5 to field agents when necessary, with the capability to fire one round per trigger pull only. I suppose that the London police adopted the same idea somehow.
Here's how they looked:
If you look closely, you can see how the London police armorers have put a wired castle nut bolt on the lower receiver so as to prohibit a policeman from swapping out the semiautomatic-only lower receiver with a full-auto lower receiver without authorization. I thought that was amusing. Anyway, I saw groups of London police walking all over the streets of London armed up like this. Piccadilly Square, the government area around 10 Downing Street near Parliament, etc. I saw more of them than the traditional unarmed police that wear stab-proof vests and the classic 'bobbie' helmet.
I got home and mentioned this to a gun group board I chat on and someone piped up and said the reason that this is so is because the London police are more comfortable with issuing carbines rather than pistols because the carbine has a more fearsome appearance, offers more accuracy over a pistol since it can be shouldered and mounted with red dot optical sights, and is not as easily lost, misplaced, or stolen as a handgun might be. So, overall: It looks tougher, shoots better, and is easier to account for than a pistol. Other than that, it doesn't perform any better or do anything different than what a holstered pistol does.
... but I guess that idea has since changed because you can see all the London police in the picture above are carrying holstered pistols.