Regarding the soundproofing spec on newbuild, you don't have to have a pre-completion test, and as long as you follow a specification which has been approved by Robust Details Ltd., and building control can see that you have followed the spec, then they sign off the dwelling without requiring you to jump through hoops. This makes the whole build a lot less scary as you dont have to budget for endless remedial sealing, reduc floors etc. Part L (thermal efficiency) is also a barrel of laughs on newbuild as of this September (new dwellings are going to have to be completely airtight). Thankfully I hear that Robust Details are working on approving some specs for this as well - fingers crossed.
Regarding doing the fitout yourself, unfortunately on a newbuild this would be impossible unless you are NHBC registered, as the flats would need the entire build to be carried out by a registered contractor in order for the warranty to be given, and for them thus to be mortgageable.
"The power of accurate observation is commonly called cynicism by those who have not got it"