What happened to Erik was disgraceful and I echo the words of Harry in not going at this in an antagonistic way after the event.
I don't know if this helps but a similar situation arose when photographing trains at stations et al. As a railway photographer, you get all sorts of crap and excuses leveled at you from "It's against the law" to " under the prevention of terrorism act...." stuff like that! However, it is not illegal to take photos on stations so the railway fraternity got together (with the help of industry magazines) with a view to clarifying the position. Network Rail (who own all the stations) and the British Transport Police eventually issued a directive that railway enthusiasts should not be challenged unless breaking the rules or not adhering to the voluntary COP i.e make your presence know to the station staff, don't use flash photography etc etc. I think carrying round an explanation of what you are doing is a good start and maybe, OSM could look at providing a registration card of some sort. This coupled with a letter to the Association of Chief Police Officers would help to get the subject of OSM'ers into the minds of the authorities. The outcome for rail photographers is that they are now encouraged to take photos as this is seen by the BTP as helping them keep rail users safe - result!
HTH