There is a plan of HMS Mersey in Battleships in Transition by Andrew Lambert. It features the hull lines, a side view and view from the stern. It is the classical plan layout, which can be often seen in plans of earlier british sailing frigates.
There are no drawing of the decks and no sailing plan.
In this book also a photo of HMS Orlando is included, which could be also found in Steam, Steel & Shellfire by Robert Gardiner (ed).
Some data and a side view of HMS Ariadne could be found in Conway's All the World's Fighting Ships 1860-1905 by Robert Gardiner (ed).
I hope that someone else knows better sources. But perhaps it would be necessary to ask at the
National Maritime Museum. If you find something additional, please tell me.
(Perhaps also someone has the link to the list of NMM's plans).