Remember not to open your door in case you get a call from an impersonator. Even if the caller makes the matter of utmost emergency, it should not coerce you into opening the door.
If you do wish to open the door, keep the chain on. This will deter the potential imposter from trying to barge in once the door has been unlocked. Keeping the chain to the door in place will only open the door so much to provide you visibility while maintaining your security.
Ask the caller for their identification. Do not rely on the ID card that they provide you (if any), rather, counter check by giving a call to the company the caller is saying he is from. Once you have gotten a confirmation from their official representative, only then should you allow the caller to enter your premises.
In case you have been a victim of an unfortunate scam and fallen prey to the tactics played by the caller, then immediately inform the Metropolitan Police London on their emergency number by dialing 999.
There are other local authorities that you can inform which are listed, with their information, according to the councils, boroughs and district councils on the following website.