If I've interpreted these sites right, there were about 6200 armed officers in 1999-2000:
http://www.homeoffice.gov.uk/pcrg/firearmstats.htm
There were about 126000 police officers in 2001:
http://news.bbc.co.uk/1/hi/uk/1411833.stm
So I guese the proportion of armed officers is a bit under 5%. I think the bulk of this will consist of armed response units, rather than routine patrols. I've never seen an armed officer on UK streets, except for in Manchester over Christmas (carrying some kind of combat rifle (?) rather than handguns) and that was only to keep a visible presence after an IRA bomb took out the city centre a few years before. To my knowledge, the only constabulary that routinely arms police officers is Nottingham and this is only for some of it's worst Beirut-styled sink estates.