iPhone sends messages by itself after sim swapping
This is one of the phone numbers used by iOS for Apple to activate a new SIM card. iOS chooses a number within the same region as the device to avoid international charges. In this case, the number is a UK number, so if you are not in the UK double-check the region of your device and SIM card.

Carriers should not charge for this message, but if you have been charged, contact the carrier and ask that they resolve the situation. A warning is displayed the very first time activation is performed, warning that ‘your carrier may charge for SMS messages used to activate iMessage’.