The concern in the UK is more over mass-produced loaves that use additives to create a sour taste and baker's yeast as the primary leavening. Some of the food chains sell them as 'artisan' loaves. There was a movement in the UK earlier to try to define what 'artisan bread' meant, they couldn't agree on it.
This is more than just a big vs little battle, the reason sourdough has a lower glycemic index is that the long time it takes for the wild yeast to produce sufficient leavening allows time for enzymes to convert much of the starch into forms that aren't absorbed as quickly. People who buy the sourfaux loaves expecting the lower glycemic index are being ripped off.