For strong white flour, look for one with a high durum (Hartweizen) wheat content. I can't remember exactly, as I got interested in these things many moons ago, but I think the durum flour has a higher gluten count, which gives more elasticity to the bread fibres. Something like that anyway.

You could try the Pasta/Knopfli flour, or try a mix of "zopfmehl", which has a higher durum wheat content than the usual white flour and can be found in all grocery shops here, together with the "Knopfli" flour.

Have fun with your bread baking.