The above answers give half the story.
The parts of Building Regs that impact your bathroom will be the Part H on Drainage and Part P on Electrical (as someone as mentioned above).
Now with Part P, one of the ways to get certification is to use a Part P registered electrician who self-certifies his work.
Part H is different - there is no self-certification. The correct way to do this is to make a Building Control application, who then come and inspect the work. With Part H it is about ensuring the sewer pipes are air-tight (so they don't pong) and flow at the right gradient (so the water-traps don't just sucked empty making the sink/loo pong.
If you are making a BC application anyway, you might just as well cover both aspects (Part H and P) together - then you wouldn't have to use a Part P registered sparky - BC would be responsible. But its up to you.
BC application for what you are doing, do not need to involve drawings - the regs are easy to comply with provided BC are informed at the righjt time (when the pipes are installed but before being boxed-in.
Phone your local BC office for friendly advice - they are there to help.
The company doing this work for you will know all the above - whether they intended to comply with it is another matter - some don't.