Can you please give me some advice?

I have recently had an extension built on my home. Internally it incomplete, it still requires plastering.

The extension is to have 4 new radiators in total, and i can do the plumbing myself.

the extension has also allowed us to have a change around inside, allowing us to have a family bedroom and an en-suite to our bedroom,
by creating 2 new stud partitions (the builder put them up when he built the extension).
there is a Heated Towel Rail to go in each new bathroom.

What i would like to know is where do i stand in completing the plumbing for the extension and the 2 new bathrooms as far as regulations etc are concerned???

Any advice, greatfully received............