UK (like most of the world) uses GSM. With GSM your identity is linked to your SIM card. Put your SIM card in another phone, and "you" are still "you" as far as the phone company can tell.
Verizon (and most of US) uses a competing "standard" called CDMA, where your identity is coupled to your phone. Which means you have to involve your phone company in order to switch phone.
Verizon (and most of US) uses a competing "standard" called CDMA, where your identity is coupled to your phone. Which means you have to involve your phone company in order to switch phone.