An architect may be retained by a client who instructs his solicitor.
The solicitor draws up a Declaration of Identity in respect of the land, relating it back to any Land Registry Documents or new Registry Maps prepared by the Architect.
The architect outlines the site on the Map, noting its area and signs it.
He also signs the Declaration.
I think that other professionals competent to read and sign off on the Map and the legals and identify areas on an OS could also swear such a document - Chartered Surveyor, Engineer, etc.
FWIW
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.