Your carrier may charge for SMS messages used to activate FaceTime and iMessage

There is no "problem" to resolve.

You are being informed that since you are using an unsupported carrier, you may be charged by them for the background SMS messages used to activate FaceTime and iMessage.

You can either go ahead and activate it, knowing you may be charged, or cancel and don't use those features.

It's your choice.

If you want to use them without being charged, then switch to a supported cellular carrier.

Your mobile network provider is not an Apple partner. That means they can charge for the international SMS messages that are used to activate and maintain your FaceTime and iMessage. Unless you want to pay for daily international text messages to Apple’s server in the UK you should log out of FaceTime and iMessage. Note that turning them off is not sufficient; you must actually log out.

FaceTime: Go to Settings/FaceTime, turn it on if it is off, tap on your Apple ID and tap Sign Out

iMessage: Go to Settings/Messages, turn it on if it is off, tap on Send & Receive, then your Apple ID, and tap Sign Out

Two more international texts will be sent after you do this, but they should then stop.

What problem?

It simply says your carrier can charge you for messaging.