That 20 quid is mostly postage from France. I think if you buy in London is is around 10 quid.
Sure, you can make it yourself, but how many people can make a loaf like that at home? How many people have a wood burning oven at home? If you were to make something of equal quality using the same methods, could you sell it for a tenner and make it worth your while to do it for a living?
While 10 or 20 quid is alot for something as common as bread, I don't begrudge anyone who has spent a lifetime mastering their art and charging anyone willing to pay to to take pleasure in consuming that art any more than I begrudge paying for the priveledge of watching a musician or stage artist, or even paying a master craftsman to do work on my house.
Well, that's my 2p anyohw