Finished floor level is finished floor level, the base of a bath or shower tray is not finished floor level.
OK. Although I thought I was being clear enough, maybe I should have spelled out the situation at greater length ....
The bathroom in question has a 'real' floor and the ceiling is about 2.8m above the 'FFL' of that. The base of the bath is about 0.2m above that floor, hence there is about 2.6m from bath base to ceiling. However, there is an area of 'raised flooring' around the bath, with its FFL about 0.6m above the 'real' floor FFL (and level with the rim of the bath), so that the ceiling is about 2.2m above that raised area. There is no shower facility in the bath.
Hence my question about 'which FFL' (really 'which floor'). If the bath had a very wide rim, that presumably wouldn't count as a 'floor'. If I had described the area around the bath as a 'surround', rather than 'a raised floor', you may not have thought of it as a 'floor' whose FFL had to be used to determine the upper boundary of Zones 1 and 2. I therefore guess that there is a 'line in the sand' somewhere, as one goes from 'wide rim', through 'narrow surround' and 'wide surround' eventually to a completely raised floor.
I thought that the intention and spirit of the regs related to what people standing in a bath or shower could touch, and what could get sprayed with water. The latter does not apply in my situation (no shower facility). As for the former, if one works from the 'raised' FFL, the ceiling is currently (just) in Zones 1 and 2 (about 2.2m). If I were to remove that surround/raised floor, the ceiling would be well outside of the zones (about 2.8m), yet absolutely nothing would have changed in terms of what could be touched by someone standing in the bath. In common sense terms, and in keeping with what I assumed to be the spirit of the regs, I don't think the ceiling should be in any zone, but it seems likely that an obsessive follower of the word of the regs would think otherwise.
Kind Regards, John