MATT iN a bathroom is known as a special location in bs7671. Depending on the zone area that your lights will be fitted , will dictate 12 volt ( selv ) or 240 v . The project site might contain info mation in the part p section .
The question is "Can I" and it will depend on the status of "I". In the main since a special location you will need to pay the LABC around £200 and hire test gear at around £75 if one is not doing a like for like replacement.
Read about Part P and then consider is it worth it. Oddly my own bathroom the low voltage lamp is where I should have extra low voltage and the extra low voltage lamps could be low voltage where they are fitted. Since all is RCD protected I am not unduly worried but if I needed to get LABC to pass the work I would have a problem.
Be careful with zones in bathrooms. If you find a site with Zone 3 then forget it as today there is no Zone 3. There will be many sites showing what can be fitted where.
Clearly you can break the law. However if you do then really asking what you can do is pointless.