is this a new installation or is this a problem that has recently started on an established system?
i don't wish to scare you but this may be serious - you need to get a professional in to look at it as you could have a potential lethal problem.
do you have a mains or gravity fed system? if gravity the thermostat may have gone on the hot water cylinder. is it making gurgling noises?
your cold water in the bathroom and toilet are probably fed from a cold water storage tank in the loft - check to see if this is the case and if so check the temperature of the water in the tank. is it warm or hot? it shouldn't be but if it is then it's almost certainly a faulty thermostat.
if the thermostaton your hot water cylinder fails it can cause the water inside to overheat and potentially boil. this will cause a rise in pressure in the cylinder that forces the hot water up through the vent pipe and into your cold water tank. if the problem persists and you have a plastic tank in the loft it could fail under the heat and burst, spilling gallons of very hot water into the room below.
there was a very sad and tragic case in the news not so long ago of a toddler that died in her bedroom when the tank in the loft above failed and she was scalded. made all the more tragic as the fault is so easy and inexpensive to fix.
i'm not saying that this is the fault in your case, it could something as simple as a hot water pipe contacting the cold feed to the toilet and bathroom and the heat is being transferred.
either way you need to take this seriously and get in a pro as soon as possible and until you've managed to isolate the fault you should turn hot water off.
no doubt you are wondering why then the cold water at the kitchen tap is not running hot. your cold water at the kitchen sink has to, by law, be fed directly from the incoming main as it is your drinking water. therefore the fault would not cause your kitchen cold to run hot.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!