I can see plasterboard would be cheaper than 18mm ply. I just went with the ply due to my previous (though limited) experience and my feeling that 12mm plasterboard would be too flexible - though I suppose it depends on the supporting wall of course.
The listing question is quite complicated and, basically, all alterations require Listed Building Consent and are considered in the context of the house itself. The important person for LBC matters is the council's Conservation Officer (CO), sometimes called an historic buildings officer or similar.
In my case, the house has different historical phases - not uncommon in old houses. My shower is part of some alterations to form a kitchen/utility room and a toilet/shower room located in the newest (18th century) part of the house, which is not deemed to be of any special historical interest. Thus, although I still needed to apply for LBC, there were no onerous restrictions except that the replacement windows had to be identical to the existing, but rotten, single-glazed ones.
Basically, the CO visited before my LBC application and we discussed my plans for the house. He advised what would be acceptable and we worked out a suitable method. We've had no real clashes, perhaps because I'm not trying to make wholesale changes to the existing 'fabric' of the house.
The main material stipulation is the use of lime mortars and renders in order to preserve the breathability of the house. Non-breathable cementitious mortar/render can seriously damage timber-framed buildings.
Otherwise, modern materials are acceptable as long as they are appropriate. For example, when I repaired the roofs, it was quite acceptable to fit a breathable membrane below the pantiles.
It might sound like a pain to be listed but a good CO is like having a free expert consultant on hand and fortunately ours has been incredibly helpful. At least the planning applications are free!
