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).
You're very correct the grinders are pretty pricey. I believe we paid about $300 for ours.
There are a few good reasons for me to have a grinder. Whether they are good reasons for you is your call.
1- I can grind whatever I want. Right now I am using hard white wheat. Unbleached, hard, white wheat flour is more expensive than your run-of-the-mill flour and comes in annoyingly small bags. I also grind beans, quinoa, oats, etc.
2- I can control the fineness of the grind. This is a minor thing but it is nice for me to be able to experiment with the texture of the product. In practice I usually leave it on the same setting but it is finer than the flour I can purchase.
3- Wheat flour is very perishable. I go through a fair bit of flour. As much as 200 oz in a heavy week. In order to have enough wheat flour on hand for a couple weeks it would have to live in the freezer. I don't want to waste that much freezer space. Whole wheat berries last almost indefinitely.
4- I like the flavor better. It tastes fresher and more nutty. This may be imagined- I haven't done any double blind tests. I should do that.
5- Cost- I had to do some of the maths as I don't usually pay a lot of attention to this.
King Arthur white wheat flour costs approximately $1.00 a lbs.
I buy 25lbs bags of bulk white wheat for $12-$15 or $0.50-$0.60 a pound.
If I use around 10 lbs of flour a week (usually a little less, it varies) I am saving about $5 a week. If you don't have access to wheat that cheap or don't make that much bread then of course the savings will be less.
As for nutrition- I read all the time that the nutrients in whole wheat flour degrade very quickly. I'm not a chemist but those results seem plausible to me based on how quickly the flour itself degrades in quality.
Best Answer
According to Waitrose:
https://www.waitrose.com/ecom/products/hovis-granary-bread-flour/476719-729900-729901
Wheat Flour (with added Calcium, Iron, Niacin, Thiamin), Wholemeal Wheat Flour, Malted Wheat Flakes (17%), Wheat Protein, Malted Barley Flour
So that's:
The protein is 14.9%, which definitely qualifies as strong.
Note also:
The own brand product is not the same:
https://www.waitrose.com/ecom/products/waitrose-duchy-strong-malted-grain-bread-wheat-flour/430555-60573-60574 - brown flour, malted wheat flakes, barley malt flour, wheat bran
We might like to compare other bread flours:
I think there is nothing at all special in any kind of way about 'Granary', it's just a brand name that Hovis license.
What you are looking at here is either brown flour (which has more bran than white flour, less than wholemeal) or a mix of white + wholemeal, malted wheat flakes, and malted barley flour
Hovis are helpfully specific about the % of malted wheat flakes - 17%. You can buy these in many countries, e.g. https://shop.kingarthurbaking.com/items/malted-wheat-flakes-2-lb
I believe that Hovis makes cheap mass-produced food as a first priority, and I believe that the malted barley flour is in fact a cheap-out. These should be malted barley flakes in fact. There are various forms of barley malt depending on whether they are sprouted or what not, but some form of barley malt will be available pretty much everywhere in the world.
Hovis protect their trademark, but 'brown bread with malt flakes' is nothing special at all, and anyone can make it....