For a variety of reasons expats can need to attest to their identity for the purposes of some administrative or legal process back in their home country. Popular reasons include the sale or transfer of a property in the UK requiring authorisation from the owner who is living abroad. A lawyer can be required to verify the sellers identity so that the sale can proceed. Also common are the validation of identity for the purpose of applying for a replacement passport or indeed for the relase of pension funds.
Sometimes this type of service is provided in the UK by a Commissioner of Oaths