My partner is in prison and is appealing against his sentence. Can the prison read mail which his solicitor sends him?

The answer to this is yes. The prison rules state that a prisoners mail, including legal mail,  may be opened if the Prisoner Govenor has reasonable cause to believe that it contains illicit enclosures, or poses a threat to prison safety or security. If the pPrison Governor believes this to be the case the prisoner is allowed to be present at the opening and must be told if any enclosure is to be read or stopped. In practice the vast majority of legal mail remans unopened.