Can a client pass over duties to an advisor to appoint a designer and if so who would sign the appointments letter and prepare the pre construction phase info

SBH

I take it the "advisor" you mention is representing the "client" in fulfilling the duties of the client role under CDM?

If so, the advisor representing the client can carry out the appointment of the designer and sign off the appointments letter on behalf of the client.

However, anyone appointing a designer to work on a project must take reasonable steps to satisfy themselves that those who will carry out the work have the skills, knowledge, experience, and, where they are an organisation, the organisational capability to carry out the work in a way that secures health and safety.

Reasonable steps will depend on the complexity of the project and the range and nature of the risks involved. This includes the advisor to the client. 

The client, or in this case the advisor to the client (unless you appoint a principal designer to undertake the role), must provide pre-construction information as soon as is practicable to every designer and contractor appointed, or being considered for appointment, to the project.

Those making the appointments must establish that those they appoint have these qualities before appointing them. Similarly, any designers or contractors seeking appointment as individuals must ensure they have the necessary skills, knowledge and experience before accepting an appointment.

I hope this helps?