You can use the apple device with any SIM Card network provider on the world.
One locked iPhone indicates this:
SIM Not Valid. The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information.
When the iPhone is connected to the computer with the iTunes open, it automatically connects to the Apple servers to check if the IMEI of your phone must be unlocked or remain locked.
Once you have purchased the unlocking, we will send you an email notifying you of the successful unlocking and you must connect the iPhone to your PC via USB cable, you will be unlocked forever.
iOS current versions NO need to connect the iPhone to the PC, just connect it to a Wi-Fi network or mobile network to check with the Apple servers if the unlocking is done.
Once your iPhone is unlocked you can use all the network providers of United Kingdom, for example 3 Hutchison, O2, Orange, T-Mobile, Tesco Mobile, Virgin Mobile, Vodafone, EE, BT Mobile and also carriers on the world.