I feel for Interceptors concerns, with the Met Police in London, just being brown is enough. It's not paranoia, the Police are quite clear and un-apologetic, the number of stops of black people is higher than whites, and results in a higher number of arrests and charges per head for blacks than whites. Therefore they say it is justified. Being in a high crime area, or being youngish and/or looking "ethnic" will increase your chances of being stopped, as will the time of day or night.
London Police are very concerned by the rise in knife crime and would rather seize items and argue about it after. I find it difficult to argue with this given the rising level of weapon violence.
No amount of licences or prior notices will change this, and indeed can increase the visits, search and seizures - e.g. licenced firearms holders. At present they do not have entry rights without a search warrant, but I hear they are quietly trying to change the law on that to enable entry and seizure of weapons based largely on unsubstantiated rumours of weapons.
Keeping stum and low profile, and polite and calm when stopped, is the only way to go. Waynes advice is right. As discussed, a meeting ticket or booking form for an event, and/or a business card, will go a long way. Something lockable in the boot of the car helps.
I suggest that membership of some clubs and organisations goes some way to prove reason. Not many of those stoppd with a baseball bat in the boot can show they play baseball regularly. I left a Bushmeet sticker on my windscreen for that very reason.