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You just need to be careful about the 'like for like' element. You need to establish that the fittings you are putting in have the correct IP rating, can you establish this from examining the existing fittings?
Problem is the existing ones could come from a pre Part P era or be over or under specified at the time of fitting. A common problem with bathroom lighting is over specification for IP ratings.
e.g. IP44 is enough for circuits outside of designated zones.
Complications like this are why no one wants to recommend an easy rule of thumb to interpret Part P.
In practice it's pretty easy to change from transformers to integrated/mains GU10 led. I think integrated offer a better choice at the moment especially regarding fitting depth. Try to get high CRI lamps and look carefully at the size and depth of hole you have to play with before ordering.
There are some new click fit fittings with wago style terminations that make the whole process pretty easy.
You are correct about individual transformers. It is normal to have a transformer per 12v bulb because the voltage drop on a tiny cable to a number of lights becomes a consideration.
If you switch to mains voltage led or integrated you may need to change any dimmer on the circuit. You can usually just change the dimmer module rather than replace the whole switch plate.
Airhead-
Thanks for the feedback @Airhead
A common problem with bathroom lighting is over specification for IP ratings.
Can I ask why that would be a problem? IP65 downlights are fairly cheap, so why not just spec them? In any case, the ceiling in my bathroom is only 220cm, so the lights there are zone 1. My understanding is that IP45 required but people usually go for IP65. My current fittings are something like this in MR16. The bulb is held in clips on the back of the bezel which has a glass faceplate and a rubber seal to the permanent fitting. Replacing the bulb therefore requires pulling the bezel off.
There's no dimmer on the circuit. I'm thinking that I'd go for integrated LED downlights; something like this, although I'll need to check the hole diameter first. It looks like they have a junction box (some with push-fit connections), so it looks like would just be a case of running a T&E tail from the existing junction box on the ring main to the new downlight. Right?
ffm
Not sure, my understanding is that like-for-like replacements aren't notifiable.
Notifiable works:
The installation of a new circuit
The replacement of a consumer unit
Any addition or alteration to an existing circuit in a special location
Since I reckon I need to replace the transformers anyway I could switch to low-voltage LEDs (the new transformers would need to be able to provide 4 or 5W, which halogen transformers can't), but it would be easier and cheaper to buy mains-voltage LED fittings with an integrated bulb.
It's very common as far as I know, although you can have a single larger transformer to power several fittings. The advantages of one transformer per bulb are (from what I've read):
More of the cabling can be smaller diameter since the the high-current side from the transformer to the bulb is shorter
The transformers are nice and small so you can fish them down through the downlight holes to replace them.
There's a junction box at every downlight that you can wire a different fitting into (which is what I'm planning to do, just need to work out if I can switch to mains-voltage fittings).