kjmpde
I would suggest attending a self build show - the NEC or similar.
Yes costs may rise but that is because people remember the cost of wood but forget the cost of and screws. Remember VAT should be reclaimable and there are items that can be bought second hand.
I envy you and wish you luck
If the architect has designed and had the plans costed, then he is likely to have used a quantity surveyor to assess the cost, so I think we can be confident he will not have budgetted for for wood but not nuts and screws.
As I understand it, Grammeretto is not intending to build the house herself, but employ a building company, so I would expect the estimate of a year to build to be reasonable as well.