The Town and Country Planning Act gives airports certain permitted development rights which should cover much of what ADP wish to do. The only thing to check is that these rights have not been surrendered for parts of the airport as part of a previous planning application. I do not know the answer to this but the potential area would be around the Western Apron which is covered bya Section 106 planning agreement as a result of the Parcelforce development (which didn't benefit the airport at all!). Below is where, generally the permitted development rights apply. It does not mention Passenger Terminals which clearly do require planning permission:-
Class
44.—(1) The carrying out on operational land by a relevant airport operator or its agent of development (including the erection or alteration of an operational building) in connection with the provision of services and facilities at a relevant airport.
(2) Development is not permitted by this class if it would consist of or include—
(3) Development is permitted by this class subject to the condition that the relevant airport operator shall consult the planning authority before carrying out any development, unless that development falls within the description in sub-paragraph (4).
(4) Development falls within this sub-paragraph if—
For the purpose of Part 14—