All distributors keep stock of the main lines - Fruit of the Loom Valueweight, Gildan 5000 etc - but as Socialgiraffe said they rely on just in time delivery from the brands for the rest. UK isn't the main/only port of entry into the EU for most brands so it needs to be imported. From conversations I have had with suppliers regarding 'out of stock' lines they tend to get a couple of deliveries a week from the major brands - hence the '5 day availability' quantities they show on their websites.