An IP44 luminaire may well
reduce the amount of warm moist air entering the roof space, but it won't eliminate it, and therefore won't provide a way to comply with Part C.
gillpeer - your plan has a number of serious flaws.
Firstly, this is notifiable work - see
//www.diynot.com/wiki/electrics:part-p, and I'm not trying to belittle you, but quite honestly if you need a wiring diagram to install a few lights you probably aren't truly competent enough to be doing it.
Secondly, since it's notifiable, there's no way you're going to be able to get Building Regulations contraventions past the BCO. It isn't just Part P that you have to comply with, it's all of the relevant regulations, and Parts C & L are immediately obvious hurdles.
If you've got a roof space over the bathroom then you will have to completely seal around the lights as you must prevent warm moist air from entering the unconditioned space because if you don't you'll get condensation up there - a contravention of Part C.
Part L problems are twofold - if there's insulation in the loft then you cannot remove it, because if you do that will make the degree of insulation worse than it was before, i.e. your degree of compliance with Part L will be less satisfactory than it was before. But you can't leave the insulation in place because the lights will require a clearance all around them to avoid overheating, so you'll need to install some kind of structure around the lights to provide clearance and to allow the insulation to be maintained.
See these for details of keeping insulation, providing space around the lights and sealing against moisture:
http://www.nhbc.co.uk/NHBCPublicati...ical/StandardsExtra/filedownload,16553,en.pdf
Pre-formed cap to seal downlights:
http://www.aico.co.uk/firecap_loftcap.htm (It's NOT a firehood....)
The second Part L issue will be the efficiency of the lights. Since you mention transformers I'm assuming the lights you plan to install use 12V halogen lamps. These will not be efficient enough - even though the Building Regulations don't actually mandate luminaires which cannot take incandescent lamps that is what Approved Document L says you have to use for a certain percentage of your lights, and rightly or wrongly many councils regard that as mandatory, so you could have a real struggle to get ELV halogens passed.
Finally the whole thing is crazy anyway. What is the wattage of the light you're removing?
How much greater is the combined wattage of the ones you plan to replace it with?
How sensible is it to deliberately make your lighting significantly less effective than it was before thus requiring a large increase in the amount you have to have in order to light the room as effectively as it was before?
(Hint - it isn't sensible at all).
Don't do it.