.50
Actually, that bullet has not been fired, there are no rifling marks on it.
Why in Essex? Well there was a war on and there were any number of US Army Air Force bases in Essex for both bombers and their fighter escorts, all of whom would have been armed with Browning .50 machine guns. The bases would have had ranges for calibrating the guns, particularly the fighters, so a fired bullet would not be at all unusual given that most of these bases have been returned to farmland.
I have a friend who farms on an old US airfield in Essex and he still ploughs up lengths of .50 ammo in belts that was put down as taxi track for the Mustangs, they had so much of it.
Now, why an unfired bullet? Any number of reasons. It could simply have been pulled from a case for some reason, or the result of a crash or whatever.
Just treat it as a little bit of history, and I won't mention Section 5.
Regards,
TonyE