Be prepared as this could happen anywhere. Before you travel practice signing in to your icloud or similar account and see what steps are required to wipe your phone. Make a note of your password and keep it somewhere separate from your phone. In a wallet or purse or with a travel companion. Then if your phone is stolen and with the shock of it happening you don't have to try and remember the password, that you haven't used in months.
Follow some of the precautions mentioned above and remember, with the millions of people visiting London the chances of it happening to you are small. It's possible but not probable.