A stock base is not the cheap option with a site-built floor as the good one. A moulded base in a square alcove of the right size is sound, quick and has almost no joints, and where the opening suits it there is little argument for anything else. The only real question is whether the room will take one, and in housing built before the 1990s it frequently will not.
Three things defeat a moulded base more often than size does. The first is square. A base is a rigid moulding with a fixed flange, and walls in an older house can run 15 to 20mm out over a 1,500mm alcove; the tapering gap that leaves gets filled with sealant doing a job it was never designed for, and it opens within a year or two. The second is the ceiling. Under a dormer or a low-slope roof, useful standing height may be 1,900mm at one wall and much less at the front, which fixes where a head can go. The third is a window inside the wet area — common here, not a problem in itself, but a detail settled before the layout is.
Size is the ordinary case and the easiest to describe. Bases arrive at 900, 1,000, 1,200 and 1,500mm; alcoves arrive at whatever the framing happened to give them. Setting a 1,200mm base into a 1,320mm opening means building a 120mm filler wall and then losing that space for good. A floor built on site takes the 1,320.
What custom actually buys is not a look. It is the head positioned where the person using it wants to stand rather than where a moulding permits, the controls within reach from outside the spray, a bench that fits, and the full width of a small room. In a 1,900mm-wide bathroom that difference is most of the comfort of the room.