Bathroom Renovation + Repiping -- Building Regs Certificate needed?

Mumili

We recently bought a house that needs a bathroom renovation and possibly repiping all the radiators (the existing ones are 10mm or 8mm microbores). I am trying to find out if I need building regulations certificate for this work. 

Bathroom: Instead of a bathtub there is a shower now, and the basin moved to a different location. The contractor tells me that there is no need for a building regulation certificate for this work. But google says if the plumbing is altered then you need a certificate. In our case, the plumbing was changed a bit due to shuffling inside the bathroom, so I am a bit confused about whether the certificate is needed. 

Repiping: The existing pipes are either 10mm or 8mm copper microbore. I am planning to get them all replaced with 15mm. I have not talked with the contractor about the certificate for this, but I think this is major work and requires some approval, no?

Doozergirl

Nope.  Neither of those require building regs sign off.  

Plumbing changes aren't affected by building regs, only when it becomes drainage and goes below ground. 

Gas, of course, is a different subject.  
 

FreeBear

Electrical work within a bathroom should be done by a competent electrician and be accompanied by the appropriate certification. Building regulations will also require you to fit adequate ventilation and an extractor. If you are stripping the tiles off the walls and end up taking the plaster off as well, depending on how much comes off, the same regulations may require you to add extra insulation.

Whether you decide to follow BR guidance is up to you. Building Control is unlikely to come round and demand that you follow their rules. But if you are stripping the exterior walls, it makes sense to put some extra insulation up, especially with the way heating costs are going.