Each light is designed and tested to a defined rating system that uses a two
digit code. This is called Ingress Protection (i.e. IP) rating. The
IP rating consists of two digits (e.g. IP65). The first digit is relates
to how
smaller an object can penetrate the casing of the light, and is largely
irrelevant for bathroom lighting (we will therefore ignore this and use
an 'x' for the first digit e.g. IPx5).
The second digit relates to water ingress and is critical to selecting
safe bathroom lights that comply with the regulations. The IP ratings are
classified
as follows:
IPx0 - No special protection
IPx1 - Protection from dripping water.
IPx2 - Protection from vertically dripping water.
IPx3 - Protection from sprayed water.
IPx4 - Protection from splashed water.
IPx5 -
Protection from projected water.
IPx6 -
Protection against powerful jets of water.
IPx7 - Protection against immersion.
IPx8 -
Protection against continuous submersion.
The bathroom zones are calculated by measuring the distance the light fitting
is from the various water sources. Your electrician will need to determine
exactly what zone each fitting is in and ensure that it meets the specific
requirement of that Zone. Below is a guide that will indicate approximately
what zone is the fitting is in:

All you need to do now is marry up the IP rating with the bathroom zone. This
is done on a minimum requirement basis (i.e. an IPx5 low voltage light
can be installed in Zones 1 but can also be installed in Zones 2, 3 as
the requirement
is less, but it can never be installed in Zone 0).
Zone 3 - IPx3 upwards
Zone 2 - IPx4 upwards
Zone 1 – IPx4 SELV 12v Class III (with transformer in Zone 3)
Most shower lights fall into the Zone 1 category, while often the rest of the downlights can be IPx0. It really depends on the design and size of your bathroom. The larger the bathroom or shower room, the more scope you have to install standard fittings. For shower cubicles and smaller wet rooms you will be restricted to Zone 1 compliant fittings.
Remember, to be sure you must consult a Part P qualified electrician.