Was in Asda (Newmains) this morning and they had deals on all their USB flash drives:
4GB ones for £4.49
8GB one for £7.49
Some 2GB ones too.
I bought the 8GB one. Maybe it's nationwide, i don't know, it may be just the one store (Newmains near Carluke/Wishaw in South Lanarkshire).
EDIT: Lots of people have found them nationwide. Some stores have not marked the low price on the shelf but they seem to scan at the reduced prices, hope this helps :)
Amazon sellers are selling the 8GB for £12 + p+p (purple) and £18(black) so £7.49 looks good to me.
Can't believe how slim it is, i'll add some pics.