General plans:

1768 Thames wherry, barge, hoy, etc. Draughts from Chapman's "Architectura Navalis - Plate 50
1800 stumpy plan/elevation
1825/50 Thames barges(topsail)
1840 Topsail Plan/elevation
1840/90 (Swimhead plans/elevation)
1860 Thames barges
1870-1895
1895 Thames barges
1895 Channel barge
1922 Auxiliary ketch barge
1925 Auxiliary ketch barge
"A Simple Thames Sailing Barge"

Sources:

"Barges" by John Leather 1984
"The Big Barges" by Hervey Benham & Roger Finch 1983.
Brown Son and Ferguson Ltd
"The CHATHAM Directory of Inshore Craft" ed Basil Greenhill and Julian Mannering.
David MacGregor Plans (Model Dockyard)
David Metcalfe

National Maritime Museum
NEXUS/MAP
"Model Boat" Magazine
Model Maker Plans Service
Planaship
Science Museum
"Spritsail Barges of the Thames and Medway" by Edgar J. March
T.B.S.C.

I'll add further information as time permits.

Plans have been available of the CAMBRIA and WOLSEY, drawn by the late L.E.Zouch.
I can't find out anything about whether they are still available from any source.
Can anybody shed any light on these plans?