Thanks for your help!
Now, sorry.
I made the letter, but the photos for her to sign should I put in the same envelope as the letter or other envelope? And also, should I tell her to sign my photos or will she know?

Hi, I wanted to ask you for help because I want to send Katie a letter with pictures to sign. The problem is that I am from Argentina and I need to know how the mail works in the UK and what information I need so that everything goes well. From already thank you very much