I have been stopped many times in London and other places, but i never considered it as being stopped or worth mentioning before as to me it is what i would expect and nothing out of the ordinary.
My job Entails measuring up buildings to prepare tenders to provide scaffolding to them and i always photograph the buildings as a record, which saves a lot of time writing down details.
I have over the years surveyed hundreds of building in London, some of them could be deemed sensitive, and to be honest i must look suspicious, taking photos of buildings that are not photogenic, crouched down on the ground with a lazer measurer etc.
Wherever possible i always go into the buildings and speak to the reception, security or even just a janitor and let them know who i am and what i am doing and show them my business card and any tender documents i have.
I have always been treated with respect and every one has been very helpful and friendly.
I have been approached many times by the police and usually explain to them what i am doing before they ask and every encounter has been proffessional and friendly, once while photographing the the main Inland Revenue building in London i was approached by 2 rough looking guys and thought i was going to be mugged, but they were not from your normal police

and showed me their warrant cards, once again no problems at all.
So all in all in my experiences the police were just doing what they are supposed to do and done it well.
Mind you have not bumped into any pco,s yet