More cheap flour

Asda are selling wholemeal (and brown) bread flour for £1 per 1.5kg bag. It's protein content is the same as  Allinson Premium Wholemeal Very Strong Bread Flour but whether this translates to the same gluten quality I don't know. If you want stoneground flour then Tesco Strong Stoneground Wholemeal flour is £1.29. If you also want it to be organic then Waitrose Organic Stoneground Strong Wholemeal Bread Flour is £1.49. This is also available online from Ocado (as well as Waitrose Deliver).

Cheap Asda flour

Asda are selling white bread flour for 50p a 1.5kg bag, which is much cheaper than any other strong flour I have seen. It also contains Ascorbic Acid (vitamin C) as a flour improver which may be to make up for it having a lower protein (and hence gluten) content than other strong white flours. For example, it is 11% protein compared to around 12% for other strong white and almost 14% for very strong white flours.