I am a size 8 too, and they never ever have enough 8s, even at Marble Arch, or online where they should have everything fgs. I got so annoyed last time I'd trailed over to Oxford Street and found nothing bigger than a 6 that I asked to see the floor manager who said that there were no 8s left 'because it's not a popular size'. I pointed out that it was so unpopular that every single one had sold in most styles, and she said, 'Well, when I say not popular...'
Every time to go to M&S, for whatever I want ranging from socks to coats for the 4 of us in our family, they never ever have all the things I want in the sizes we want. And I go to a very big M&S in Bromley.
Last time I went I wanted to buy uniform: 2 cardigans, 6 polo shirts, 2 skirts, 3 pairs of trousers, a pair of shorts, and a 5 pack of boys underpants.
I came away with 0 cardigans, 2 polo shirts, 1 skirt, 1 pair of trousers, 0 shorts, 0 underpants. Just because they didn't have any/adequate stock in my dc sizes (10 years and 6 years).