You are trying to solve a problem that you don't have. You can't use a UK phone number. And you don't need one!
Just sign up with Gmail or Apple. It is rubbish in the UK, but it will be useful to text girls when in Thailand. The QR code thing works well there. Touch your picture, press the tiny QR code at the top, and you have a code bar girls can scan.
Or do the other way round. Press the person icon with the plus, press the QR code icon (under the words 'add friends') and you will get a camera to scan her QR code. You can also add TF girls with their Line ID.