Is it OK for who to change the switch? Under the Part P regulations bathrooms are notifiable so it would cost around £200 to DIY likely cheaper to use an electrician who is a member of a scheme and can self certify.
As to BS7671:2008 the bathroom is split into 4 zones and areas. 0, 1, 2 for zones and 3 meters horizontal to have a 13A outlet (not really for bathroom it's really for when a shower is fitted in a bedroom).
Zone 2 is 0.6 meters from Zone 1 and it's this distance which would normally restrict using a switch on the wall. 701.55 does list exceptions which includes (viii) Luminaires. But I would consider that to be the light it's self not the switch. It's common to have light switch outside the bathroom.
There may also be a problem with RCD protection? Any new circuit in a bathroom will need RCD protection so if the lights are not already on a RCD it could be a problem.
Of course people do break the rules but you are asking if your allowed so I assume you want to stay within the law. So if more than 0.6 meters from where you want the switch and the lights are RCD protected then worth getting an electrician to move it. However if within 0.6 meters just forget the idea or have the switch outside the bathroom. If no RCD still can be done but will cost more so is it worth it?
