When you activated your device on another network, when you attempted to activate iMessage and/or FaceTime, the iPhone will send a hidden SMS to the servers in the UK for activation. If your carrier charges for those, and/or blocks those, then you are not using a supported carrier. Apple supported carriers do not charge for those. If they are blocking them, then the phone will continue to send them until they get a response. You will have to turn off iMessage and FaceTime. The fact you are being charged must be taken up with your carrier. That is not something Apple is responsible for.