How to make your own Granary® flour from existing ingredients?
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:

white flour, with vitamins for no culinary reason
wholemeal flour
17% malted wheat flakes
wheat protein because bread needs strong flour (high in gluten)
malted barley flour

The protein is 14.9%, which definitely qualifies as strong.
Note also:

62.9% carbs (of which 1.7% sugar), 1.8% fat, 7.5% fibre, 14.9% protein, hence 12.9% moisture/ash

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

68.4% carbs (of which 3.2% sugar), 1.6% fat, 4.3% fibre, 13.5% protein, hence 12.2% moisture/ash

We might like to compare other bread flours:

white - 67.4% carbs, 14.9% protein, 2.8% fibre https://www.waitrose.com/ecom/products/waitrose-white-bread-flour/006224-2744-2745
Canadian very strong wholemeal - 57.0% carbs, 15.8% protein, 11.7% fibre https://www.waitrose.com/ecom/products/waitrose-stoneground-bread-wholemeal-flour/051804-25784-25785
brown - 72.3% carbs, 12.7% protein, 6.1% fibre https://www.waitrose.com/ecom/products/essential-strong-brown-bread-flour/666913-94660-94661

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....