The BBC lists strong white flour as simply flour made from hard white wheat, which tends to have a higher gluten content.
By mixing all purpose flour (German: 550, French: 55) with the package recommended amount of vital wheat gluten or by using bread flour you should be able to make bread in your machine just fine. I usually use either AP flour or a mixture of AP and whole wheat (German: 1600, French: 150), or even all whole wheat (although with all wheat I find adding gluten is extremely helpful for getting a well-risen loaf).
You can also buy other types of high gluten flour, either with that label or listed as bread flour.
If you live in a part of the world with "0" and "00" flour, the protein content of "00" flour is similar to that of all purpose flour, but the grind of all purpose flour will be more coarse. "Panifiable" 00 flour has the gluten content of bread flour (source).
Yes, and it is very easy. I do it all the time.
You only need a very simple calculation. You don't even have to be precise.
If you do want precision, you will have to find out 1) how much of your flour protein is gluten, 2) how much of your "vital wheat gluten" is gluten, and 3) how much gluten content you need for your recipe. Then use a simple rule-of-three calculation to get the amount needed to add.
I usually don't bother, because 1) and 2) is information which is very hard to find, and due to the large difference in gluten weight and complete weight, imprecision from not using true proportions is minimal. What I do is:
- Look up the protein content of your flour (usually printed on the package), for example 9.6 grams per 100 grams
- Look up the gluten content needed for your bread recipe. If it is not specified, 12.5% is usual for bread flour.
- Add the difference in vital wheat gluten. In the example above, add 2.9 g of vital wheat gluten per 100 g of flour.
This doesn't produce exactly 12.5% gluten content, but I think that it is within the tolerance of most recipes; indeed, not all commercial flours are exactly 12.5%, they vary with brand and season.
I add the powder to the flour and mix it well before making the bread. If I am using a preferment, I add all the gluten to the preferment and make the non-fermenting part with AP flour only, so my gluten can benefit from longer autolysis.
I have no direct comparison with "true" bread flour, as I have never used it. But my breads requiring bread flour turn out good for my standards. There is no problem with bad distribution, the dough turns out very smooth and evenly elastic. There is a pronounced difference to using AP flour only.
Best Answer
"Strong flour" and "Bread flour" generally mean the same thing -- lots of gluten, so the dough can stretch and incorporate lots of bubbles.
Not all bread demands high-gluten flour, but the traditional airy loaf of Western Europe and most of the USA does.
If what you're making is bread of that kind, "strong bread flour" is what you want.
If you're making a cake or a sauce, things will probably be fine -- try it. It might not be perfect but it won't be a disaster. It's worth having some less strong flour (often labelled as "plain flour") in your store cupboard.