In the UK a mobile phone is stolen every 12 seconds (it was around one every 3 minutes a few years ago). If your phone is lost or stolen you can call your network operator and have them block the SIM card. However, the handset is still usable.
To block the handset you need your IMEI (International Mobile Equipment Identity). which you get by typing the following into your handset: *#06#
However, it’s currently down, and has been since yesterday morning.
In the meantime, make a note of your IMEI somewhere.