Looking at your photos, I've no idea what is going on here as its wrong on so many levels.
But I'll hazard a guess.
What you have is a structural suspended slab. It is not "ground-bearing" as the beams support the load of the floor. Two beams together are supporting the load of a wall which is sort of standard.
I am amazed that you have some stone fill in between the beam as this will puncture the DPM. It appears the stone fill is to temporary shuttering supporting the concrete screed over the beams.
Again amazed there is no mesh reinforcement in this concrete.
So the crack is definitely a structural issue and claimable under your NHBC warranty.
If fact there are many breaches in warranty standards (which the NHBC deem are guidance when you claim!)
You should try and get hold of the working drawings and specifications, especially the structural engineering drawings. These should be publicly available at whoever approved them for Building Control.
You may not get to keep them but you should be able to see them.
If you have difficulty do a
subject access request. Taylor Wimpey did use a system of suspended floor, with polystyrene in between the beams.
Given your home is only 5 years old it is questionable how it could meet building regulations for insulation without any floor insulation!
Finally, it is best practice that any gas service pipe is formed in steel barrel pipe, painted with two coats of bitumen paint and wrapped in 'denzo' tape.
Plastic sleeved copper pipes are not suitable for gas service pipes in this location.