Why do people have a suspicion that a world war two era bomb by the british would be nuclear?
I think their concern is to not get everyone near the bomb killed or cause significant damage to a section of highway.
After looking up all the possibilities, I believe the bomb was a Tall Boy:
It was designed to penetrate through soil and concrete and then dentonate from within an underground structure. My guess is that the crew had to abandon and chose the Highway to drop it on, however on impact the bomb failed to detonate and got covered up by rain water run off an mud.