One of the first and probably most obvious precautions that one can take is keeping your mobile phones hidden unless absolutely necessary. If you are walking alone, take your cell phone out only when there is an important call that needs to be attended to.
Putting your phone in the front pocket or inside a bag is the safest bet. In such a case, even if an attempt of robbery is made, you can repel the attack. Besides, they are good hiding places for your phone.
There is a system in place by major mobile phone networks that a phone can be blocked within 48 hours if it is lost or stolen, rendering it absolutely useless. But this can only be done if a phone has been registered with them.
Mobile phones can also be registered with the National Mobile Phone Register. This will allow you to immediately inform the police and other organizations, including second-hand trade shops.
In case you wish to personally inform the Metropolitan Police Department, they can be contacted by dialing 101 for non-emergency and 999 for emergency issues.