The basis for bread flour prices is a complex subject but I'll try to explain.
The major flour millers in the UK have kept the price of flour high for many years. Flour from British wheat has been at a premium for many years as has flour from "hard" wheat such as that imported from Canada to make some UK strong flour. The crisis in the Ukraine has has a massive effect on flour prices as wheat flour from Eastern Europe has always been reasonably priced and has been used to manufacture everything from 50p loaves in supermarkets to the more premium priced loaves.
The major flour millers in the UK claim that they use only British grown wheat and the costs involved are higher than that from imported wheat. The wheat varieties are also said to be better and thus more expensive, than wheat grown elsewhere. This is reflected in the price you pay for their flour. To be honest, they are being economical with the truth. Bulk purchasers who get their wheat from these same major millers such as your local bakery, pay a fraction per kilo of the price that you pay even after taking necessary supermarket profit into account.
There are millers in the UK who use the best wheat to make their flour and soil-association certified organic flour is also available. These include Shipton Mill and Marriages amongst others and good quality bread flour is still available on these and other websites. They tend to supply in 16kg bags, I think. Give them a try as I don't think you'll look back.