99% of shoe shops now are Fast Moving Consumer Goods shops, so they only stock sizes that are common and styles that won't stay unbought in the basement. That was always the case with the British Shoe Corporation ( Lilley & Skinners, Saxone etc) I worked in Lilley & Skinner in the Kings Rd during my summer holidays in '66 when the World cup was on in London, all of our stock then was mostly the common sizes and they turned over fast and got replaced fast.
Anything odd that got left over used to go on a special double commission ticket that was called oddly enough, a Spliff, it had a big S overprinted on the ticket.
As mentioned above, the money was good, I was clearing £15 a week as a school kid! None of my mates had anything like that kind of money on their holiday jobs.
Additionally the majority of shoes that they do stock will fall apart in a week if you actually take them out of the box and do something rash like walking in them.
That applies particularly to womens shoes if you take them back, the staff will inform you that 'they are a fashion shoe and that you must have danced/walked/ran/played sports or swiveled excessively in them'.
Even the majority of 'Trainers' are not sports shoes as it is a Style Description; I was told that by a spotty scrote in a large retail sports store after a pair of 'Trainers' I had bought while on a visit to the UK, failed before my two weeks were up.
As for those of us who exhibit Anatidaean traits when shoebuying, it's a bitch.
Luckily in Spain I seem to have less problems finding wide shoes than I used to in the UK, perhaps the Spanish have a national trait of wider feet, I have also found that my feet seem to be getting wider as I get older, I started adult life as a UK 8, now I am a 9 but my feet are still the same short stubby langth they were before, the only benefit being if I drop a heavy object on the end of my shoe, my toes are nowhere near it.