IMEI stands for International Mobile Equipment Identity. Every mobile device sold has one — a 15-digit number that identifies the hardware, regardless of which SIM is inside.
How to find your IMEI
Dial *#06# on any phone. It also appears on the original box and (on most devices) under Settings → About.
Why it matters
Carriers use IMEI to block stolen phones from their networks. If your phone is reported stolen, the IMEI goes onto a shared blocklist and no UK network will let it connect — even with a new SIM.
The IMEI is separate from the EID (which identifies your eSIM chip) and the ICCID (which identifies the SIM profile).