If you want to help a prisoner manage their financial affairs while they’re in prison, the prisoner will need to complete a Power of Attorney form. Once completed, this will give the named person permission to liaise with the relevant agencies (landlords, banks, creditors, etc.) on the prisoner's behalf. The power of attorney form should be completed using the accompanying guidance notes. 

If the person in prison overseas is unable to have contact with the outside world and therefore to sign a Power of Attorney, it may be that the Guardianship (Missing Persons) Act 2017 would be relevant for the purposes of appointing someone to manage their affairs. There is more information in the Code of Practice and further advice would need to be sought from a lawyer.