Wearing a stab vest is just the kind of dumb thing politicians tend to do for image/publicity reasons. I sincerely doubt she's actually wearing it for protection.
It might also need considering that she had to wear it under some regulation or other (as I think police officers have to). In either case, I still don't think it was out of fear for her safety.
On the more interesting wider issue of 'are our streets safe?'
this chart from the British Crime Survey (which takes into account estimates on unreported crime) indicates that overall crime has decreased in recent years.
this country-comparison by NationMaster lists the UK as 18th out of 60 for recent murders (which isn't very good but if you look at the numbers it's much better than others on the list. Plus consider the size of several of the comparison countries).
Rather less positively, however, NM does rank us 3rd out of 60 in 'Total Crimes'.
The Home Office site also testifies to a rise in crime, but it needs to be remembered that the HO only considers convictions - it doesn't take into account unreported crime. So it actually testifies to a higher rate of conviction.
So the evidence indicates that while things aren't exactly great on the crime front, they're not nearly as bad as they could be or are made out to be.