As the following explanation indicates:
Strong Flour

This type of flour should be made from hard wheat varieties and produces elastic dough because it has a high gluten and protein content. Gluten is rather like chewing gum and can hold the carbon dioxide gas produced during dough fermentation to produce a good crumb structure in the finished loaf.

Hard wheat varieties produce the best flour for bread making. However the weather in the UK sometimes makes it difficult for farmers to grow hard wheat in quantity. Unlike many other flour millers, at Doves Farm Foods we blend home grown varieties with imported hard wheat for optimum bread making performance. We do this rather than fortifying the flour by adding refined gluten as in many other mills.