The Law Society lays out the difficulties for criminal law firms from the new requirement to meanstest in the attached pdf document.
They also lay out the legal issues for solicitors should they collectively withdraw or restrict their work in order to protect their own interests.
Competition Act 1998 applies to such collective agreements.
However they then appear to lay out the reasoning that will justify refusal to accept instructions.
"For example, some of the protocols work on the basis that defence solicitors will accept instructions for new clients limited in the first instance only to progressing the legal aid application. While solicitors are as a matter of ethics generally free to decide whether or not to accept instructions from a particular client (see Guide 12.01) and are generally free also not to act until funding is secure (whether the client is to be legally aided or privately paying), these are general principles and should be applied to the facts of each individual case. In each individual case the acceptance of initial instructions on a limited basis will establish a relationship of solicitor and client and so the question of whether or not the limitation is in the interests of that individual client will always have to be considered. If all the solicitor is prepared to offer a client is such a limited retainer and this will not serve the interests of that particular client, then the proper course ethically for the solicitor may be to decline to accept instructions in the first place."