They didn’t reject the bidders because of the materials, they were rejected due to the overall bid price. They had a budget (£8 million iirc), most came in at ~£11 million except the winning bid which was at ~£9 million. They then engaged in some shady pre negotiation with them to reduce their bid before accepting it.

After accepting it the contractor realised some of their values were wrong so in addition to attempting to find hundreds of thousands of pounds of saving from the council, they were also trying to save hundreds of thousands due to their own misestimates.

Part of this led them to select aluminium cladding over the originally specified zinc cladding which was much less flammable but much more expensive.