To disable a stolen mobile phone.
To find serial no. key in *#06#, a 15 digit no. appears. keep this safe. if your phone gets stolen phone provider, give them this no. The phone will be blocked. you won't get your phone back but thief will not be able to use it.
The IMEI can be read from the label under the battery, usually. The thief will probably flog the mobile to a fence who will ship them to a foreign country where they won't be blocked. You may get some satisfaction from knowing a casual thief won't be able to use it in the UK. They may con someone in the pub to buy it though.