I like how you give an extreme example of a serious crime as a pre-text to sway the debate in your favour.
http://letyoutellit.com/2010/07/03/16-year-old-boy-shots-and-kills-2-police-officers-with-a-ak-47/
Legalise guns for all as suggested in this thread and we get examples of the above video.
Also arming police with guns can equal http://www.chicagotribune.com/news/local/breaking/chibrknews-9yearold-boy-shot-on-southwest-side-20110520,0,7347575.story
Shock stories don't justify debates either way, more fact's and statistics are needed here on all sides, rather than just assumptions based on bias opinion's (Seriously The amount of North american's in here, commenting on the need for the police to have guns without knowing much, if anything about the way the UK is, is laughable)
Still, No to officers carrying guns, specially trained officers are on standby and are able to respond to incidents much much quicker than regular officers anyway, if a regular officer meets a perp with a gun by chance, that's chance alone, and he must call back-up for back off. When responding to a call about an incident involving firearms, special police will be there double quick.
And although I can't be bothered to re-search statistics, I'd be willing to venture more criminals would actively seek to carry fire-arms if they knew they would be confronted with them.