Mixture:
36 years in the telecoms industry including BT Wholesale & BT Retail, now with an independent supplier. I can assure you that your paper thin walls are very robust indeed. I suspect that, if you've never worked within the BT Group it is hard to comprehend the independence of the businesses that make up that group these days.
Anyway, no-one has yet explained why no other network supplier has been able to commit to developing the fibre network alongside/in competition with Openreach.