US and UK laws on confidentiality differ. In the UK material that a reasonable recipient would consider confidential (or expressed to be confidential), not in the public domain, and disclosed in circumstances of confidentiality (in fairly general terms) will be protected as confidential.

In the UK it is settled law that while the property in a physical letter passes to the deliveree upon delivery, the copyright in the contents remains with the sender. Mick Jagger has used this to prevent publication of his love letters to Shrimpton (but it looks as if Marsha Hunt, the inspiration for "Brown Sugar", may have outflanked the notoriously mercenary Jagger by selling Jagger's love letters to her in the USA).