that's not a correct picture.
Per Electrical regulations, Zones are measured from a fixed bath or fixed shower.
The pic also shows a zone round the basin. I believe this is an invention by the association of bathroom tilers, or some such, who are not a competent body with discretion to lay down electrical regulations.
Zone 3 does not exist any more, it was in old versions of the regs. A thing that is not in zone 0, 1 or 2 is outside the zones.
A thing that is more than 2250mm above the floor is outside the zones, so you could put a crystal chandelier on the ceiling if you wanted to, and it didn't dangle within 2250mm of the floor.
Here are some better pics and explanations.
Yes, ceiling height is relevant. A thing that is I would not force anybody to put a light switch near the basin, or a vanitory unit, or a bidet or a WC, but the picture is not an accurate representation of the zones.
Electrical sockets must be at least 3metres from a fixed bath or shower, which in most UK bathrooms you won't achieve.
Things must be suitable for where they are put, which IMO means not where the hairdryer can fall into the basin, or your weeing child can spray the electric heater, or the shower spray can squirt at the light switch. Yes, you can have a light switch in a bathroom.
Since the zones are measured from a fixed bath or shower, a room which does not contain a fixed bath or shower is not a bathroom so does not have zones (e.g. cloakroom).