Porcelain wants a base that does not flex; on a suspended timber floor that usually means extra work, and vinyl over levelling is the simpler route.
A Birmingham bathroom carved out of a bedroom in an inter-war semi sits on joists, and the choice there is a rigid vinyl tile from the BML range over a levelled, sealed deck: light, waterproof at the tile itself, and unbothered by the flex of a timber floor under a full bath. Where the bathroom is on the slab of a ground-floor extension, porcelain from the tile range has the rigid base it needs.
The Berry Alloc panels close the shower with interlocking boards and no grout, and in the small bathrooms of the Kingstanding and Weoley Castle semis that is also the surface easiest to keep. In the Perry Barr and Smithfield flats the lease governs the build-up and the weight, which is another reason the vinyl tile is the usual answer above ground level.
Semi-detached houses are 36% of the Birmingham stock, and the semi's bathroom is upstairs on a timber floor, often over the kitchen. The floor over joists is checked for level and stiffness before a tile goes on it, because a tile over a floor that flexes cracks at the grout; the rigid vinyl floats over it and is the more forgiving of the two on timber.
Terraced houses are 28% of the homes in Birmingham, and a terraced bathroom in Birmingham is often a later addition, on the ground floor behind the kitchen or in a first-floor extension over it. A timber first floor under a bathroom is the case for the rigid vinyl, which floats and is light; a ground-floor slab takes either floor once the reading says it is dry.