Bread flours are made from wheat flours and contain gluten, which when mixed with water creates an elastic consistency which binds the bread dough together. Kneading the bread dough stretches the gluten throughout the dough, trapping the CO2 given off by the yeast causing the dough to rise.
When baking bread we always recommend using bread flour. The lack of gluten in plain flour will cause the CO2 to escape so the resulting loaf will be plain and small compared to a loaf made with bread flour.
Allinson have a fantastic range of bread flours which have been expertly milled. Please see www.allinsonflour.co.uk for more information.