Your photos appear to contradict your description.
The photos would appear to comply with disabled access required in part M of the building regulations.
But if you bought the property on the basis the configuration was different to what you now have, ie shown in the brochure or show home working drawings shown to you all before reservation, you can claim you were misinformed which is against consumer law.
As for whether it is a big job or not, it is wrong. I can understand that perhaps the floor will need to be taken up and drainage re positioned to change the configuration. That said, perhaps Miller would offer you a settlement amount to accept it as it is, saving them the cost of the work and administration headache and you the mess and inconvenience.
Perhaps £1,000 - £2,000 would be enough?
http://www.brand-newhomes.co.uk/makeadonation.htm