From what you describe this is classed as timber frame construction.
There are always detailing issues, especially as most designs feature areas cladded with timber, other sheet materials, render or vertical tiling.
If the junctions where these meet brickwork or window and door openings that, if they are not detailed correctly, then rain water can and will get in.
As you have found, once the timber frame is wet, it cannot really dry out quickly.
It has (or should have) a vapour barrier on the inside behind the plasterboard and a breather membrane on the cavity side.
The timber frame should have a moisture content of no more than 20% before it is boarded.
Any water ingress or water from plumbing leaks will saturate the frame. This will gradually travel down (by gravity) to the sole plate that the timber frame wall panels are supported on and fixed to.
If the sole plates are rotting it is very serious.
All structural timber should be pressure preservative treated whcih will give some protections against rot.
Any cut ends of timbers must be treated on site. (they never are!)
Apart from the
fire risk, this is another reason to avoid
timber frame construction for new homes.
It is a possibility that should be considered for Taylor Wimpey take your rotting homes in part-exchange.
You could always sell it on straight away, you may even make some money whilst it is being built.
At least then you would be rid of a defective new home that no one will buy and as a bonus, you would have handed back the problems and cost to Taylor Wimpey.
Any coverage in the media would be helpful, especially if you can get the TV involved.
I have also started a Facebook Group
"Unhappy New Home Buyers" that you are welcome to join.
"Modern methods of construction" is a recipe for disaster.
House builders have struggled for years to get the basics of 'traditional construction' right to a reasonable standard. Now with new technology and techniques they won't have a clue!