About the present incident in Heathrow and the quoted 747 accidents (El Al and others), one has to understand that U238 is used in quite many industrial processes.
U238 is NOT fissile but it is radioactive (half-life: ~4.5 billion years). It decays through an alpha radiation which is hardly detectable but its child product is the short-lived Th234 which has a half live of 24 days and decays with a beta- radiation (ie: electrons). This beta- radiation triggers radioactivity detectors.
U238 is used for photographic intensifiers, ceramic colorants, dental porcelain additives, armor–piercing bullets, specialised anvils, ballast (as in B747), radiation shields (yes, it looks contradictory, but it is efficient), etc.
It is quite possible that a disused material from one of these above activities has been mixed with other high-value scrap metal.
It is public knowledge that many international airports are equipped with radiation detectors; if this metal was intended for a malicious use, it would not have transited through Heathrow.