The answer is found readily in Rule 41 of the Solicitors' Professional Conduct and Practice Rules which says that a solicitor must not allow the solicitor's business name or stationery to be used by a mercantile agent in a manner likely to mislead the public. The Rule succinctly gives guidance to solicitors who are acting for mercantile agents both in respect of the relationship between the solicitor and the mercantile agent and the manner of communication between the mercantile agent and the client's alleged debtor. The communications of a debt collection or mercantile agent to an alleged debtor should not improperly suggest they have emanated from a solicitor's office. The solicitor must maintain full control over any actions taken in his or her name. A fairly common enquiry is whether it is improper for a solicitor to act for a relative or person with whom he or she has a close personal relationship.
Some people automatically assume it is improper.
It is not necessarily. However it is inadvisable, probably for two main reasons. Firstly, the solicitor may be insufficiently objective to act in the client's best interests and may become too personally involved in the matter, not demonstrating the professional detachment which solicitors should be able to display in most effectively carrying out their duties to their clients. This may be a particular danger if the client is a person who may readily exercise some domination over the solicitor. This may seem unusual or unlikely.